A reference glossary of terms encountered across essential oil literature. Each definition is short and factual, intended as a quick lookup while reading oil pages, constituent pages, or safety material.
Entries are grouped into five categories: chemistry, toxicity, skin and topical, regulatory and certification, extraction and production. Within each category, terms are listed alphabetically.
An essential oil is a mixture of dozens of volatile organic compounds. Most belong to the terpene and terpenoid families, alongside phenylpropanoids and a smaller share of oxygenated compounds.
| Term | Definition |
|---|---|
| Alcohol | |
| Aldehyde | Compounds with a -CHO group at the end of a carbon chain. Examples include citral, cinnamaldehyde, and benzaldehyde. Aromatic, prone to oxidation, and some can irritate or sensitize skin. |
| Alkane | A saturated hydrocarbon containing only single C-C and C-H bonds. Rarely a major essential oil constituent. |
| Alkene | A hydrocarbon with at least one C=C double bond. Most terpenes fall into this group. |
| Alkyne | A hydrocarbon with a C≡C triple bond. Very uncommon in essential oils. |
| Aromatic compound | |
| CAS number | A unique identifier assigned by the Chemical Abstracts Service. Used to reference a specific chemical unambiguously, independent of trade names. |
| Chemotype (CT) | A variant within a single plant species that looks identical but yields an oil with a different dominant constituent. Named by the main compound, for example thyme CT thymol or thyme CT linalool. |
| Chirality | The property of a molecule whose mirror image is non-superimposable, typically from a carbon bonded to four different groups. The two mirror forms are enantiomers and may differ in aroma and biological activity. |
| Enantiomer | One of the two mirror-image forms of a chiral molecule. The two enantiomers of limonene illustrate this: the (+) form smells of orange, the (-) form of pine. |
| Ester | The product of an acid reacting with an alcohol, carrying a -COO- group. Common in essential oils as linalyl acetate, geranyl acetate, and methyl salicylate. Usually mild on skin with fruity or floral aroma. |
| Ether | |
| Furocoumarin | A compound made of a furan ring fused to a coumarin. Several furocoumarins, including bergapten, psoralen, and methoxsalen, are phototoxic. See the phototoxicity page. |
| GC-MS | Gas chromatography mass spectrometry. The standard technique for analysing essential oil composition: GC separates constituents, MS identifies each one. |
| Isomer | Two or more compounds sharing a molecular formula but differing in structure. The E/Z (trans/cis) notation marks substituent positions across a double bond; R/S marks the absolute configuration of a chiral carbon. |
| IUPAC name | The systematic chemical name set by the International Union of Pure and Applied Chemistry. Describes a molecule's full structure through its name. |
| Ketone | |
| Lactone | A cyclic ester. Coumarins and sesquiterpene lactones such as alantolactone belong here. Some lactones are skin sensitizers. |
| Molecular weight | Molecular mass expressed in g/mol. Linked to volatility: smaller molecules evaporate more readily. |
| Monoterpene | |
| Monoterpenoid | |
| Oxidation | A loss of electrons, typically driven by exposure to oxygen, light, and heat. Oxidized oils produce hydroperoxides and breakdown products that raise the risk of skin irritation and sensitization. |
| Oxide | A compound with oxygen inside a ring; the most common example is 1,8-cineole. Found in eucalyptus, niaouli, rosemary, and many other oils. |
| Peroxide | A compound containing an -O-O- bond. Hydroperoxides form as essential oils oxidize and are the main cause of sensitization from aged oils. |
| Phenol | |
| Phenylpropanoid (phenylpropene) | |
| Phenylpropene ether | A phenylpropanoid subgroup carrying a methoxy or methylenedioxy group on the aromatic ring. Methyleugenol, estragole, and safrole all belong here. |
| Polymerization | A reaction linking small molecules into long chains. In aged oils, terpenes may polymerize, producing resin and thickening the oil. |
| Racemic mixture | A 50:50 mixture of two enantiomers, written (±). It does not rotate plane-polarized light. Natural essential oils are rarely racemic because biosynthesis favours one enantiomer. |
| Refractive index | The ratio of the speed of light in vacuum to its speed in the oil, measured at a standard temperature. One of several indices used to check identity and adulteration of essential oils. |
| Sesquiterpene | A 15-carbon terpene (C15H24) built from three isoprene units. Examples include β-caryophyllene. Heavier and slower to evaporate. |
| Sesquiterpenoid | |
| Specific gravity | The ratio of the oil's density to that of water at the same temperature. Most essential oils are lighter than water (< 1); a few, such as clove and cinnamon, are heavier (> 1). |
| Terpene | A hydrocarbon built from multiples of the isoprene unit (C5H8). The basic carbon skeleton of most essential oil constituents. |
| Terpenoid | A terpene that has been modified, typically oxygenated. Includes monoterpenoids, sesquiterpenoids, and diterpenoids. |
| Terpene hydrocarbon | A pure-hydrocarbon terpene with no oxygen. Examples include limonene, pinene, and myrcene. Prone to oxidation in air. |
Most toxicity terms come from pharmacology and toxicology. Essential oil work borrows these measures to describe safe ranges for individual constituents or whole oils.
| Term | Definition |
|---|---|
| Abortifacient | A substance that induces abortion or terminates pregnancy. Some oils high in sabinyl acetate or pulegone have been used this way and carry significant toxicity. |
| ADI (Acceptable Daily Intake) | The maximum amount of a substance that can be ingested daily over a lifetime without appreciable harm, expressed as mg/kg body weight. Set by bodies such as JECFA and EFSA. |
| Anti-androgenic | A substance that reduces the activity of male hormones (androgens). May affect reproduction and sexual development. |
| Anti-estrogenic | A substance that opposes estrogen activity, typically by blocking receptors or inhibiting synthesis. |
| Antioxidant | A substance that slows oxidation by neutralizing free radicals. In essential oils, phenols such as thymol and carvacrol show antioxidant activity. |
| Carcinogenic | Capable of causing cancer or promoting tumour growth. Methyleugenol and safrole are examples among phenylpropene ethers. |
| Choleretic | A substance that stimulates bile production by the liver. Some essential oil ketones show choleretic activity, with risk in hepatobiliary disease. |
| Dermatotoxic | Causing harm to skin, whether through irritation, corrosion, necrosis, or sensitization. |
| Embryotoxic | Harmful to the embryo in early pregnancy, potentially causing miscarriage or malformation. |
| Estrogenic | Showing estrogen-like activity, typically by binding estrogen receptors. Phytoestrogens from hop and lavender oils have been examined for this effect. |
| Fetotoxic | Harmful to the fetus after organ formation. May affect growth, function, or viability. |
| Genotoxic | Damaging DNA or chromosomes. An early warning for mutagenic and carcinogenic potential. |
| Hepatotoxic | Harmful to the liver. Pulegone, menthofuran, and some phenols can damage liver tissue with prolonged exposure or high doses. |
| IC50 | Inhibitory Concentration 50: the concentration at which a substance inhibits 50% of a given biological activity (enzyme, receptor). Used to compare potency between compounds. |
| LD50 | Lethal Dose 50: the dose that kills 50% of test animals, expressed as mg/kg body weight. Lower values indicate higher toxicity. |
| LOAEL | Lowest Observed Adverse Effect Level: the lowest dose at which an adverse effect is seen in a study. Used to derive safe limits for humans. |
| MoE (Margin of Exposure) | The ratio between a reference dose (such as the BMD or NOAEL) and the actual human exposure. Higher MoE means greater safety margin. |
| MTD | Maximum Tolerated Dose: the highest dose a test animal can tolerate without severe toxicity. Used in chronic studies. |
| Mutagenic | Capable of causing mutations in genetic material. The Ames test is a standard screen for mutagenicity. |
| Nephrotoxic | Harmful to the kidneys. Some α-pinene-rich oils or oxidized terpenes have been reported to irritate the kidneys at high doses. |
| Neurotoxic | Harmful to the nervous system. Camphor, thujone, and pinocamphone are essential oil examples that can trigger seizures at high doses. |
| NOAEL | No Observed Adverse Effect Level: the highest dose at which no adverse effect is seen in a study. The main basis for deriving ADI and dermal limits. |
| Oncogenic | Capable of initiating cancer. Often used synonymously with carcinogenic, with focus on activating cancer-related genes. |
| Photoallergic | An allergic skin reaction caused by interaction between a chemical and UV light through the immune system. Unlike phototoxicity, it requires prior sensitization. |
| Photocarcinogenic | Capable of causing skin cancer when combined with UV exposure. Several furocoumarins fall into this category. |
| Photosensitizing | Making skin more reactive to light, leading to phototoxic or photoallergic responses. |
| Phototoxic | Causing direct skin damage on UV exposure without prior sensitization. Furocoumarins in bergamot and some citrus oils are the main cause. See the phototoxicity page. |
| Sensitizer | A substance that triggers an immune response after exposure, leading to allergic contact dermatitis on later contact. Skin sensitizers and respiratory sensitizers exist as separate categories. |
| TDI | Tolerable Daily Intake: the maximum daily exposure to a contaminant or non-intentional substance without appreciable risk. Used for substances not deliberately added to food. |
| Teratogenic | Causing congenital malformations when exposure occurs during pregnancy. Sabinyl acetate in savin and mugwort is an essential oil example. |
Most essential oil use is topical. The terms below describe skin responses, safe concentration ranges, and the standard test methods used to evaluate irritation or sensitization potential.
| Term | Definition |
|---|---|
| Allergic contact dermatitis (ACD) | Skin inflammation caused by an immune response after sensitization has occurred. Appears on later contact with the allergen, often delayed 24 to 72 hours. |
| Berloque dermatitis | Drop-shaped pigmentation on skin from applying furocoumarin-containing oils followed by sun exposure. Most often linked to bergamot. |
| Comedogenic | Capable of blocking pores and forming whiteheads and blackheads. Most pure essential oils are non-comedogenic, but carrier oils and base waxes may be. |
| Dermal limit | The recommended maximum concentration of an oil or constituent in a leave-on product, expressed as a percentage. Bergamot, for example, has a 0.4 % limit for sun-exposed leave-on products. |
| Dermatosis | A general term for any disease of the skin, whether inflammatory or non-inflammatory. |
| Draize test | An animal skin and eye irritation test, typically on rabbits, applying the substance and grading the response. Increasingly replaced by in vitro methods. |
| Eczema | A chronic inflammatory skin condition with itch, redness, dryness, and sometimes cracking. Includes atopic, contact, and dyshidrotic forms. |
| Emollient | A substance that softens skin by filling gaps between corneocytes, improving feel and smoothness. Carrier oils are typical emollients. |
| Hapten | A small molecule that binds to body proteins to form a complex able to trigger an immune response. Most essential oil sensitizers act through a hapten mechanism. |
| Human Maximization Test (HMT) | A human sensitization test using high concentrations and maximizing methods to detect allergenic potential. Now less common on ethical grounds, replaced by HRIPT. |
| Humectant | A water-binding ingredient that draws moisture from the air and lower skin layers to the surface. Examples include glycerin, hyaluronic acid, and propylene glycol. Essential oils are not humectants. |
| Irritant contact dermatitis (ICD) | Non-immune skin inflammation caused by direct damage to the stratum corneum. Phenols, aldehydes, and oxidized oils are common triggers. |
| Keratin | The main structural protein of the stratum corneum, hair, and nails. Disulfide bonds give it mechanical strength. |
| Occlusion | A state where the skin surface is sealed, slowing water evaporation and increasing transdermal uptake. Applying essential oils under occlusion substantially raises exposure. |
| Occlusive | An ingredient that forms a film on the skin surface to block water loss. Petrolatum, beeswax, and some heavy esters are typical occlusives. |
| Patch test | A test where a small amount of substance is applied to the skin (typically back or forearm) for 24 to 48 hours to check for irritation or allergy. A basic step before using a new product. |
| pH | A scale of acidity or alkalinity from 0 to 14. Healthy skin sits around pH 4.5 to 5.5. Most leave-on cosmetics are formulated in the 4 to 7 range. |
| Photo-patch test | A patch test combined with UV exposure, used to detect phototoxic and photoallergic responses. Two identical patches are applied; only one is irradiated. |
| Repeat Insult Patch Test (RIPT) | A human sensitization test, often written HRIPT (Human Repeat Insult Patch Test). Multiple applications over several weeks check both induction and elicitation. |
| Sensitization induction | The first stage of allergic contact dermatitis, when the immune system learns to recognize an allergen. Once induced, a person will react on subsequent exposure. |
| Sensitization elicitation | The later stage, when a sensitized person re-encounters the allergen and develops dermatitis. The elicitation threshold is typically lower than the induction threshold. |
| Skin penetration | The movement of a substance from the skin surface into deeper layers and possibly the circulation. Influenced by molecular size, lipid solubility, concentration, and skin condition. |
| Stratum corneum | The outermost layer of the epidermis, made of dead corneocytes and intercellular lipids. The main barrier against water loss and external penetration. |
| Transepidermal water loss (TEWL) | The amount of water that evaporates through the epidermis, measured in g/m²/hour. Elevated TEWL indicates barrier damage. |
Essential oils and cosmetics operate within a layered system of regulation and voluntary certification. The bodies listed below set either safety rules or origin and process labels.
| Term | Definition |
|---|---|
| CITES | Convention on International Trade in Endangered Species. An international treaty regulating trade in threatened species. Appendix I bans trade, Appendix II requires permits, Appendix III applies by country request. Relevant for rosewood, sandalwood, and agarwood. |
| COSMOS | COSMetic Organic and natural Standard. A European organic and natural cosmetic certification administered by an alliance of five bodies (Ecocert, Soil Association, ICEA, BDIH, Cosmebio). |
| Demeter | A certification for biodynamic farming and products, stricter than standard organic. Established in 1928. |
| Ecocert | A French certification body, one of the first to certify organic and natural cosmetics. Now operates under the COSMOS standard. |
| EFSA | European Food Safety Authority. The EU body for scientific risk assessment of food safety and additives, including essential oil flavour compounds. |
| EU Cosmetics Regulation 1223/2009 | The principal EU cosmetics law, in force since 2013. Requires a product information file, safety assessment, allergen labelling, and bans animal testing. |
| EU 26 allergens | A list of 26 fragrance compounds that must be declared on cosmetic labels in the EU above threshold (0.001 % for leave-on, 0.01 % for rinse-off). Includes linalool, limonene, citral, eugenol, geraniol, citronellol, coumarin, and others. |
| EU SCCS | Scientific Committee on Consumer Safety. The EU scientific body advising on the safety of cosmetic ingredients (successor to SCCP and SCCNFP). Its opinions feed into regulation. |
| Fair for Life | A fair trade certification covering products and supply chains for both food and non-food categories, including essential oils and cosmetics. |
| Fair Trade USA | A US fair trade certifier. Guarantees minimum prices, community premiums, and labour standards for producers. |
| GRAS | Generally Recognized As Safe. A US FDA classification for food additives accepted as safe through expert consensus. Many essential oils hold GRAS status for flavouring use. |
| Health Canada Cosmetic Ingredient Hotlist | A Health Canada list of substances prohibited or restricted in cosmetics sold in Canada. Updated periodically. |
| IFRA | International Fragrance Association. The fragrance industry body that sets voluntary safety standards for fragrance materials in consumer products. IFRA Standards are updated through periodic amendments. |
| IFRA Categories 1–11 | An 11-category product classification used to set fragrance concentration limits. Cat 1 covers lip products, Cat 5A body lotion, Cat 9 rinse-off, Cat 11 non-skin contact. IFRA limits for each material vary by category. |
| ISO 4730 | The ISO standard for tea tree oil (Melaleuca alternifolia). Specifies percentage ranges for 15 main constituents, including terpinen-4-ol ≥ 30 % and 1,8-cineole ≤ 15 %. |
| JECFA | Joint FAO/WHO Expert Committee on Food Additives. The international body evaluating safety of food additives and flavour compounds, including many essential oil constituents. |
| NaTrue | A Brussels-based natural and organic cosmetic label. Three tiers: natural, natural with organic portion, and organic. |
| Organic certification | The audit and certification process for products grown without synthetic pesticides, GMOs, and with minimal chemical inputs. Requires at least three years of land conversion and annual audits. |
| Pharmacopeia (Ph. Eur., BP, USP) | Official compendia setting quality standards for pharmaceutical raw materials, including medicinal essential oils. The three main ones are Ph. Eur. (European Pharmacopoeia), BP (British Pharmacopoeia), and USP (United States Pharmacopeia). |
| USDA Organic | The US organic certification under the National Organic Program. Four label tiers: 100 % organic, organic (≥ 95 %), made with organic (≥ 70 %), and ingredient-list mentions. |
Extraction method determines the chemical signature of the final aromatic product. The same plant can yield an essential oil, an absolute, a CO2 extract, or a hydrosol, each with a distinct composition.
| Term | Definition |
|---|---|
| Absolute | An aromatic product obtained from a concrete by ethanol washing and solvent evaporation. Used for delicate flowers such as jasmine, rose, and tuberose. Thick, intense, and closer in scent to the fresh flower than the distilled oil. |
| Attar (atr) | A traditional Indian and Middle Eastern aromatic, distilled by capturing flower or plant material in a sandalwood oil base. The sandalwood holds and carries the aroma. |
| Carrier oil | A plant-based base oil used to dilute essential oils before topical application. Examples include jojoba, sweet almond, and fractionated coconut. Unlike essential oils, carrier oils are non-volatile. |
| Cold expression | Mechanical pressing of citrus peel without heat. Yields an oil that retains natural furocoumarins and is therefore typically phototoxic. Examples include bergamot, lemon, and lime. |
| Concrete | An intermediate from non-polar solvent extraction (typically hexane) of flowers. Contains both aromatic molecules and plant waxes, with a near-solid texture. Used directly in perfumery or processed further into an absolute. |
| Distillation | A method that separates volatile compounds by heat and condensation. Steam distillation passes steam through plant material; water distillation boils material in water; combined distillation does both; hydrodiffusion routes steam from the top down. |
| Enfleurage | A traditional technique using animal or vegetable fat to absorb scent from fresh petals. Flowers are replaced repeatedly until the fat is saturated, producing a pomade that is later processed into an absolute. Now rare outside small artisanal work. |
| Essential oil | A mixture of volatile organic compounds from a single botanical source, obtained by distillation or by cold expression for citrus. Insoluble in water, partially soluble in carrier oils. |
| Expression | The mechanical pressing process used to obtain essential oil, almost exclusively for citrus peel. Synonymous with cold expression when no heat is applied. |
| Extraction yield | The ratio of essential oil obtained to plant material weight, expressed as a percentage. Rose petals yield roughly 0.015 %, eucalyptus leaves 1 to 2 %. |
| FCF (Furocoumarin-Free) | A citrus oil version with furocoumarins (mainly bergapten) removed by fractional distillation or filtration. No longer phototoxic. Bergamot FCF is the most common example. |
| Fractional distillation | Distillation through a tall fractionating column to separate constituents with similar boiling points. Used to remove unwanted compounds, produce FCF oils, or isolate a specific constituent. |
| Hydrodiffusion | A distillation variant where steam enters from above the plant material and moves downward. Allows shorter extraction times and higher yields with some materials. |
| Hydrosol (hydrolat) | The aqueous phase from steam distillation, carrying small amounts of water-soluble aromatic compounds. Low concentration, suitable for direct skin use. Hydrolat is the same product, more common in French usage. |
| Isolate | A single pure constituent separated from an essential oil or made by chemical synthesis, used in perfumery. Examples include linalool isolate and eugenol isolate. |
| Oleoresin | A solvent extract of a spice or resin, containing both essential oil and non-volatile resin and pigment fractions. Highly concentrated, used in food flavouring. |
| Pomade | An intermediate from enfleurage: fat saturated with aroma after repeated petal contact. Can be used directly or further processed into an absolute. |
| Rectification | A refining process where an essential oil is redistilled, typically fractionally, to remove impurities, toxic constituents, or unwanted compounds. |
| Resinoid | A solvent extract from dry plant resin such as benzoin, labdanum, or myrrh. Thick and used as a fixative in perfumery. |
| Solvent extraction | A method using chemical solvents to dissolve and separate aromatic compounds. Hexane is common for flowers, producing concrete then absolute. Supercritical CO2 uses carbon dioxide above its critical point and leaves no solvent residue. |
| Spagyric | A traditional alchemical extraction combining distillation, fermentation, and calcination to recover mineral salts. Rare in modern commercial essential oil work. |
| Tincture | An alcohol-based (ethanol) extract at varying concentrations and material ratios. Common in herbal medicine and traditional perfumery, distinct from pure essential oil. |