Glossary of Essential Oil and Aromatherapy Terms
Plain definitions of essential oil and aromatherapy terms, from absolute and hydrosol to chemotype, phototoxicity and GC-MS, with sources and notes on terms with no official meaning.
Content last updated . This page has not been medically reviewed.
Safety first. Natural products can cause side effects and interact with medicines. Read essential oil safety before you use anything on this page.
Safety first. Essential oils are strong and are not for swallowing. If anyone may have swallowed one, call Poison Help at 1-800-222-1222 (US); call 911 for collapse, a seizure or trouble breathing. [1] More is on the safety quick reference.
Words on bottles, in studies and in advertisements are often used loosely. This glossary says what each term means in the sources we read, and says so plainly when a term has no official definition. It is information only. It does not suggest using any oil, and it gives no amounts.
How oils are made and what they are called
| Term | What it means in the sources we read |
|---|---|
| Essential oil | A concentrated plant extract that keeps the natural smell and flavor of its source, obtained by mechanical pressing or distillation. Synthetic oils are not considered true essential oils. [2] The FDA says there is no regulatory definition for "essential oils". [3] |
| Steam distillation | Steam is passed through plant material; after cooling, the oil separates from the water and can be collected. [4] A review calls it the most common method overall. [5] |
| Hydrodistillation | A related method, grouped with steam distillation in the research. Both leave an aqueous by-product called a hydrosol. [6] |
| Expression (pressing) | Physical crushing of the oil glands in a fruit rind. It is the method most used for citrus peels. [5] Pressed and distilled citrus oils can differ: one aromatherapy association lists expressed lemon and expressed lime as photosensitizers and distilled lemon and distilled lime as non-phototoxic. [7] |
| Solvent extraction | Taking scent compounds from plant material with an organic solvent. A review calls products made this way "aromatic extracts" rather than classical essential oils. [8] |
| Enfleurage | Transfer of the oil from flower petals to fat. [5] |
| Concrete | A solvent-extracted aromatic material. In one study, obtaining the concrete was the first step toward an absolute. [9] Concretes may contain waxes, pigments and resins together with the scent compounds. [8] |
| Absolute | Made from a concrete by cold extraction with ethanol to remove precipitated fractions. [9] Absolutes are grouped with concretes and resinoids as aromatic extracts. [8] See absolutes, hydrosols and infused oils. |
| Resinoid, oleoresin | Listed with concretes and absolutes as solvent-extracted aromatic extracts. [8] |
| CO2 extract (supercritical) | A method that may produce volatile-rich fractions but, depending on conditions, can also recover non-volatile oily material. [8] |
| Hydrosol (hydrolat, floral water) | The water-based by-product of distillation. It holds only traces of essential oil (usually less than 1 g/L) and water-soluble compounds. [6] Its chemistry usually differs from the oil's. [6] |
| Fragrance oil | Not a term the sources define. NIEHS says synthetic oils are not considered true essential oils. [2] See essential oils vs fragrance oils. |
| Carrier oil | The base oil in which essential oils are diluted for a massage. [10] Poison Control says diluting in a carrier oil is the best way to avoid a bad reaction from direct skin contact. [1] See carrier oils explained. |
What oils contain
| Term | What it means in the sources we read |
|---|---|
| Volatile | Easily evaporating. A review calls essential oils complex mixtures of volatile plant chemicals. [8] NCI says the aroma fades quickly when the oil is left open to air. [10] |
| Constituent | One chemical inside an oil. Each oil has a unique mix, and the mix can vary within the same plant species or from plant to plant. [2] |
| Terpene, terpenoid | Chemical families. A review says essential oils are mainly terpenes, terpenoids and phenylpropanoid or benzenoid compounds. [8] |
| Common constituents | NIEHS names eight chemicals found in most of 93 oils analyzed, including limonene, linalool and eucalyptol. [2] |
| Chemotype | A version of one plant species defined by which chemicals dominate its oil. Spearmint oils grouped into a carvone-rich type and a pulegone/menthone type. [11] Basil plants showed five types, including linalool and methyl chavicol. [12] A review says oil composition depends on species, chemotype, plant organ and growing conditions. [8] |
| Menthol | A chemical in peppermint oil. NCCIH says it should not be inhaled by or applied to the face of an infant or small child. [13] |
| Methyl salicylate | The main chemical of wintergreen oil, which is 98% methyl salicylate. It is also in many over-the-counter creams, liniments and medicated oils for aches. [14] |
| Eugenol | The chemical named in a case report of liver failure after a toddler swallowed clove oil. [15] |
| Camphor | A chemical used as a moth repellent and in skin products. Even a small amount is dangerous if swallowed. [1] |
| Furocoumarin, psoralen | Chemicals that make skin react to sunlight. Bergamot oil has them, mainly one called bergapten. [16] Lime, celery, parsley, figs and carrots also contain psoralens. [17] |
Quality, testing and labels
| Term | What it means in the sources we read |
|---|---|
| Botanical (Latin) name | The genus and species of the plant. NCCIH lists tea tree oil under the Latin name Melaleuca alternifolia. [18] Labels with only a common name can leave the plant unclear, so see reading a label. |
| GC-MS | Gas chromatography coupled to a mass spectrometer. It identifies the components of an oil from the pattern of fragments they produce. [19] See GC-MS testing explained. |
| Adulteration | Common forms are adding cheaper oils or synthetic materials. [20] Diluting an oil with vegetable oil is another, and it can leave the usual marker percentages unchanged. [20] |
| "Pure" | Not defined in the sources we read. Poison Control says it can be hard to know exactly what is in the bottle: which plant species, how much active ingredient, or whether there are contaminants. [1] |
| "Therapeutic grade" | A marketing phrase. We found no definition of it in the sources we read. See the claim checked. |
| "Natural", "organic" | The FDA has no regulations defining either word for cosmetics. [21] |
| "Fragrance" on a label | US rules let fragrance and flavor ingredients be listed simply as "Fragrance" or "Flavor". [3] |
| Oxidized oil | An oil that has degraded with time or exposure. NCI says the compounds in oils can change as they get older and many oils have no expiration date. [10] NCCIH says tea tree skin reactions may be more likely if the product is old or exposed to heat, light or air. [18] For tea tree oil, oxidation increases its allergenic potency. [22] |
Reactions of the skin and body
| Term | What it means in the sources we read |
|---|---|
| Irritant (dermal irritant) | Something that causes an immediate skin effect, blotchy or red and sometimes painful. The severity depends on the concentration applied. [7] The association's examples include cinnamon bark, clove bud, oregano and thyme. [7] |
| Sensitization | A type of allergic reaction. The first exposure may show little or nothing; later exposure to the same or a similar material can cause a severe inflammatory reaction. [7] Once it happens, the person is most likely sensitive to that oil for many years. [7] |
| Patch test (clinic) | A dermatology test for allergy. In one clinic network, 908 of 10,930 patients (8.3%) reacted to at least one of 12 oils tested. [23] Those patients had skin problems, so this is not a rate for the public. |
| Patch test (skin check) | A small-area skin check suggested by an aromatherapy association when someone may have sensitive skin. [7] See patch testing and stopping a reaction. |
| Phototoxicity, photosensitization | A skin reaction that needs both the oil and ultraviolet light. Reactions range from a mild color change to deep weeping burns. [7] The FDA notes that certain citrus oils used safely in food can be harmful in cosmetics on skin exposed to sun. [21] See citrus oils and sunlight. |
| Phytophotodermatitis, berloque dermatitis | Sun-triggered skin reaction from psoralen-containing plants. Berloque dermatitis is a variant caused by fragrances rich in psoralens, most commonly oil of bergamot. [17] |
| Mucous membrane irritant | An oil with a heating or drying effect on the lining of the mouth, eyes, nose and reproductive organs. [7] |
| Idiosyncratic reaction | An unusual, hard-to-predict reaction to a commonly used oil. [7] |
| Aspiration | Oil entering the lungs, for example when someone chokes while swallowing it. It can cause pneumonia. [1] |
| Hepatotoxic | Harmful to the liver. A case report says clove oil has important hepatotoxic effects. [15] NIH's LiverTox says pennyroyal oil taken by mouth is highly toxic and linked to liver injury and death. [24] |
| Endocrine disruptor | A substance that interferes with the normal function of the body's hormone system. NIEHS says lab work found that lavender and tea tree oils, and some of their chemicals, showed hormonal activity in test-tube cells. [2] |
| Prepubertal gynecomastia | Breast tissue growth in boys before puberty. Clinical cases have linked it to skin use of essential oils, not to aromatherapy. [2] |
| Interaction | Essential oils may interact with medicines. [1] Ask a pharmacist; see medication interactions. |
Practice terms
| Term | What it means in the sources we read |
|---|---|
| Aromatherapy | The use of essential oils from plants as a complementary health approach, most often by inhaling them or applying a diluted form to the skin. [25] |
| Indirect and direct inhalation | Indirect: breathing in an oil spread by a room diffuser or placed on a tissue. Direct: using an individual inhaler. [10] See inhalation aromatherapy. |
| Diffuser | A device that spreads an oil through the air. [10] See diffusers. |
| Reed diffuser | Sticks that absorb scented liquid and release it into the room. The liquid is an essential oil dissolved in isopropyl (rubbing) alcohol. [1] |
| Aromatherapy massage | One or more oils diluted into a carrier oil and massaged into the skin. [10] See aromatherapy massage. |
| Dilution | The share of oil in a carrier. The severity of an irritation reaction depends on the concentration applied. [7] This site does not give dilution charts; see why charts differ. |
| Topical | Applied to the skin. [18] |
| Blend | Two or more oils used together. In a trial of a blend, an effect cannot be pinned on one oil. |
Names that cause confusion
| Name | The trap |
|---|---|
| Chamomile | Two plants: Roman, Chamaemelum nobile, and German, Matricaria chamomilla, both accepted names on GBIF. [28] NCCIH says there are two types, German and Roman. [29] |
| Cinnamon | NCCIH names Ceylon cinnamon, Cinnamomum verum, and cassia cinnamon, Cinnamomum aromaticum. [30] |
| Orange | The aromatherapy association lists expressed bitter orange as a photosensitizer and sweet orange among non-phototoxic citrus oils. [7] GBIF treats Citrus sinensis as a synonym of Citrus × aurantium. [28] |
| Pennyroyal | LiverTox names Mentha pulegium. [24] GBIF also accepts a different plant, American pennyroyal, Hedeoma pulegioides. [28] |
| Pine oil | A cleaning-product ingredient and a plant oil are not the same thing. Case reports of poisoning involve household cleaners that contain pine oil. [31][32] |
| Eucalyptus | The EMA names Eucalyptus globulus, E. polybractea and E. smithii as sources of the oil it assessed. [4] |
What this glossary does not do
It does not define amounts, schedules or blends, because this site gives none. It does not list "grades" of oil, because the sources we read describe no official grading. For one oil at a time, see the at-a-glance table; for safety words in general, see the safety glossary.
Frequently asked questions
What is the difference between an essential oil and a fragrance oil?
NIEHS says essential oils come from pressing or distilling plants and that synthetic oils are not considered true essential oils. [2] The sources we read do not define "fragrance oil" more closely.
Is "therapeutic grade" an official category?
We found no definition of it in the sources we read. The FDA says there is no regulatory definition for "essential oils" at all. [3]
What is an absolute?
It is an aromatic extract made with solvents. The study we read describes an absolute as made from a concrete by cold extraction with ethanol. [9]
Does a GC-MS report show an oil is safe?
GC-MS identifies the chemicals in an oil. [19] The sources we read do not describe it as a safety test, and a plain composition check can miss dilution with vegetable oil. [20]
What does phototoxic mean?
It describes a skin reaction that needs both the oil and ultraviolet light. [7] A journal report describes blistering after bergamot aromatherapy oil and later UVA exposure in a tanning salon. [16] The FDA notes certain citrus oils can be harmful in cosmetics on skin exposed to the sun. [21]
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
Regulation and research terms
| Term | What it means in the sources we read |
|---|---|
| Cosmetic and drug | The FDA decides by intended use. A product meant only to cleanse or make a person smell good is a cosmetic; one meant to treat or prevent disease, or to affect body structure or function, is a drug. [21] Claims such as easing pain, treating anxiety or helping sleep are drug claims. [21] See the law and essential oils. |
| GRAS | "Generally recognized as safe", an FDA label for ingredients in food and fragrances. NCI says most essential oils carry it. [10] The FDA notes that some citrus oils used safely in food can be harmful in cosmetics on skin exposed to the sun. [21] |
| Traditional use (EMA) | An EU category where trial evidence is insufficient but the effect is plausible and the product has been used safely for at least 30 years. [26] |
| HMPC | The EMA's Committee on Herbal Medicinal Products, whose conclusions summarize the medicinal uses of a herbal substance. [4] |
| Feed-additive opinion (EFSA) | Several European opinions assess oils as animal-feed additives. Their lines on irritation and sensitization describe how to treat the oil when handling it. [27] |
| Preclinical | Research in cells or animals. See human vs lab research and human, animal and laboratory research. |
| Evidence labels on this site | Established, promising but limited, mixed, insufficient evidence, no good evidence of benefit, evidence of harm, preclinical, traditional use only, marketing claim. See evidence labels. |
References
- Poison Control (National Capital Poison Center). Essential oils: Poisonous when misused. Read 2026-10-23.
- National Institute of Environmental Health Sciences (NIEHS). Essential Oils. Last reviewed March 19, 2026. Read 2026-10-23.
- U.S. Food and Drug Administration. Fragrances in Cosmetics. Read 2026-10-23.
- European Medicines Agency (EMA). Eucalypti aetheroleum - herbal medicinal product. Read 2026-10-23.
- Review. Essential Oils as Natural Sources of Fragrance Compounds for Cosmetics and Cosmeceuticals. Open-access review (PMC). Read 2026-10-23.
- Review. Unlocking the Potential of Hydrosols: Transforming Essential Oil Byproducts into Valuable Resources. Open-access review (PMC). Read 2026-10-23.
- National Association for Holistic Aromatherapy (NAHA). Safety. Read 2026-10-23.
- Review. Essential Oil Quality in Medicinal and Aromatic Plants: Linking Plant Biosynthesis, Matrix Selection and Extraction Strategy. Open-access review (PMC). Read 2026-10-23.
- Study. Terpenoids in the Essential Oil and Concentrated Aromatic Products Obtained from Nicotiana glutinosa L. Leaves. Open-access article (PMC). Read 2026-10-23.
- National Cancer Institute. Aromatherapy With Essential Oils (PDQ). Read 2026-10-23.
- Isfendiyaroglu H et al. Molecules. 2024. Chemical Characterization of the Essential Oil Compositions of Mentha spicata and M. longifolia ssp. cyprica. (abstract read via PubMed). Read 2026-10-23.
- Barut M, Tansi LS. BMC Plant Biol. 2026. Chemical profiling and chemotypic diversity of basil (Ocimum basilicum L.) genotypes. (abstract read via PubMed). Read 2026-10-23.
- National Center for Complementary and Integrative Health (NCCIH). Peppermint Oil. Read 2026-10-23.
- Davis JE. J Emerg Med. 2007. Are one or two dangerous? Methyl salicylate exposure in toddlers. (abstract read via PubMed). Read 2026-10-23.
- Janes SE et al. Eur J Pediatr. 2005. Essential oil poisoning: N-acetylcysteine for eugenol-induced hepatic failure and analysis of a national database. (abstract read via PubMed). Read 2026-10-23.
- Kaddu S et al. J Am Acad Dermatol. 2001. Accidental bullous phototoxic reactions to bergamot aromatherapy oil. (abstract read via PubMed). Read 2026-10-23.
- Wang L et al. Cutis. 2002. Berloque dermatitis induced by "Florida water". (abstract read via PubMed). Read 2026-10-23.
- National Center for Complementary and Integrative Health (NCCIH). Tea Tree Oil. Read 2026-10-23.
- Review. Fundamental Chemistry of Essential Oils and Volatile Organic Compounds, Methods of Analysis and Authentication. Open-access review (PMC). Read 2026-10-23.
- Capetti F et al. Molecules. 2021. Adulteration of Essential Oils: A Multitask Issue for Quality Control. Three Case Studies. (abstract read via PubMed). Read 2026-10-23.
- U.S. Food and Drug Administration. Aromatherapy. Read 2026-10-23.
- de Groot AC, Schmidt E. Contact Dermatitis. 2016. Tea tree oil: contact allergy and chemical composition. (abstract read via PubMed). Read 2026-10-23.
- Geier J et al. Contact Dermatitis. 2022. Contact sensitization to essential oils: IVDK data of the years 2010-2019. (abstract read via PubMed). Read 2026-10-23.
- NIH LiverTox (NIDDK). Pennyroyal Oil. 2020 entry. (abstract read via PubMed). Read 2026-10-23.
- National Center for Complementary and Integrative Health (NCCIH). Aromatherapy. Last updated January 2020. Read 2026-10-23.
- European Medicines Agency (EMA). Lavandulae aetheroleum - herbal medicinal product. Read 2026-10-23.
- EFSA FEEDAP Panel. EFSA J. 2025. Safety and efficacy of a feed additive consisting of an essential oil derived from the leaves of Pogostemon cablin (patchouli oil) for use in all animal species. (abstract read via PubMed). Read 2026-10-23.
- GBIF Secretariat. GBIF Backbone Taxonomy: species name matching for the 47 plant names in the oil table and glossary. Queried 2026-10-23. Read 2026-10-23.
- National Center for Complementary and Integrative Health (NCCIH). Chamomile. Read 2026-10-23.
- National Center for Complementary and Integrative Health (NCCIH). Cinnamon. Read 2026-10-23.
- Welker JA, Zaloga GP. Chest. 1999. Pine oil ingestion: a common cause of poisoning. (abstract read via PubMed). Read 2026-10-23.
- Brook MP et al. Ann Emerg Med. 1989. Pine oil cleaner ingestion. (abstract read via PubMed). Read 2026-10-23.
Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.