Essential oils are leading natural solutions for a variety of ailments because of their potency and proven properties and benefits. With their increasing popularity among the general public and health professionals, more people are seeking factual information that can help them use them safely and effectively. So, what are essential oils? It turns out defining what an essential oil is is much harder than you think.
Traditionally, the aromatherapy industry has held to the following definition, likley adopted from an earlier definition used by the flavor and fragrance industry:
The National Association for Holistic Aromatherapy (NAHA) defines essential oils as highly aromatic substances made in plants extracted by distillation.  They further explain that essential oils can be extracted by steam distillation, hydrodistillation (also called water distillation), or expression.  The NAHA prefers to call substances extracted by enfleurage, solvents, and carbon dioxide (CO2) as extracts rather than essential oils. Interestingly, the NAHA alludes to accepting CO2 extracts as essential oils when they state “An essential oil’s chemical make-up may vary from the plant from which it was extracted from due to its method of extraction; for example, distillation vs. carbon dioxide extraction vs. expression.”
Another industry organization, the Alliance of International Aromatherapists (AIA) does not have an official definition that I could find on their website. 
However, some scientists and scientific organizations allow for a broader definition:
“An essential oil is a product made by distillation with either water or steam or by mechanical processing of citrus rinds or by dry distillation of natural materials. Following the distillation, the essential oil is physically separated from the water phase.” is a definition that closely matches that given by the NAHA for an essential oil by the International Organization for Standardization (ISO). 
A 2012 review article published in The Journal of Essential Oil Research states that essential oils can be obtained from plants by “hydrodistillation, solvent extraction, cold pressing, and supercritical fluid extraction (CO2).”  The article also notes that essential oils are most commonly extracted by steam distillation.
Similarly, a 2018 review article from the Asian Journal of Green Chemistry includes a variety of extraction methods for essential oils, including steam distillation, hydrodistillation, solvent extraction, supercritical fluid extraction, instantaneous controlled pressure drop process, microwave-assisted extraction, and ultrasound assisted extraction.  The article further states that the traditional methods of extraction (steam distillation and hydrodistillation) have several drawbacks, which led to the emergence of newer processes to “optimize the performance of the essential oil in both quantitative and qualitative terms.”
Another review article from 2015, uses a simple definition that essential oils are “a mixture of saturated and unsaturated hydrocarbons, alcohol, aldehydes, esters, ethers, ketones, oxides phenols and terpenes, which may produce characteristic odors” and “extracted from the flowers, barks, stem, leaves, roots, fruits and other parts of the plant by various methods.” 
A study even specifically calls a CO2 extract a “supercritical essential oil.” 
While another study considers CO2 extraction as an innovative technique to extract essential oils allowing for the extraction of additional essential oils that cannot be obtained with traditional methods. 
A recent symposium (Essential Oil Symposium for Medical Professionals) I attended that included some of the foremost and respected researchers and clinicians on essential oils defined essential oils as “distilled or extracted from a single botanical variety [where] nothing is added or taken away.” This means no addition of natural isolates, synthetic petrochemicals, or cheaper essential oils. Supercritical CO2 extracts can certainly meet this definition because they do not add any residual solvent.
Even Josh Axe, DNM, CNS, DC weighed in on the subject during an interview with Shape Magazine, defining essential oils as “highly concentrated compounds extracted from plant parts using a steam-distillation, cold pressing, or CO2 extraction process.”
Why must we hold to such a limited definition of essential oils—oils obtained only by distillation or expression—when CO2 “select” extraction can produce an end-product very similar to—sometimes even superior to—traditionally distilled essential oils without leaving residual solvent? For example, CO2 bergamot can produce an oil similar to expressed bergamot, distilled caraway closely resembles CO2 caraway, CO2 lemon and expressed oils composition are comparable, CO2 tea tree produces an oil similar to distilled oil but with less loss of bioactive constituents due to thermal degradation, and many more essential oils could be cited.
Indeed, I recently asked a group of trained aromatherapists to review the complete composition of 10 essential oils and tell me whether it was a distilled or expressed essential oil or one extracted by CO2. Trained aromatherapists were only able to correctly identify the extraction method about 50% of the time, which is not statistically different than guessing really. This emphasizes that many CO2 oils look just like traditionally extracted essential oils when you compare compositions.
In addition, some essential oils extracted by CO2 may be more therapeutic due to the preservation of bioactives. For example, supercritical German chamomile select CT bisabolol oxide essential oil contains similar levels of bisabolol oxide A (50.4% to 56.8%), bisabolol oxide B (0.2$ to 21.5%), alpha-bisabolol (1.5% to 8.9%), and sometimes minor amounts of chamazulene (0.0% to 1.8%)—some of the key active constituents in the oil identified by research—while preserving some matricine (up to 3.5%). This is highly desirable because matricine is significantly more anti-inflammatory than chamazulene.
Another example is supercritical ginger select essential oil. Gingerols and shogaols are pungent compounds found in ginger roots with proven anticancer, antiobesity, antiallergic, antioxidant, and anti-inflammatory properties. These compounds are not present in distilled ginger essential oil but the selectivity and better extraction properties of CO2 allow these health-promoting constituents to appear in high levels in CO2 ginger essential oil while still maintaining other key bioactives (e.g. alpha-zingiberene, beta-sesquiphellandrene).
Instead, I propose that we accept as essential oils any plant extract that produces an end-product similar to traditionally distilled essential oils without introducing a foreign substance (such as animal fat or solvents) during the extraction process. This would allow select CO2 oils, distilled oils, and expressed oils to each be called essential oils being delineated as such: supercritical lemon essential oil (select), distilled lemon essential oil, and expressed lemon essential oil. Absolutes would not be included as an essential oil because they leave trace to small amounts of solvent in the end-product.
One argument against using CO2 essential oils in clinical practice has been the lack of safety information because of the presence of these additional constituents. However, I performed a literature review on the safety of these constituents and list additional cautions in my book “SuperCritical Essential Oils” based on these new constituents. For the clinician, the name and definition are not as important as reliable efficacy and safety. Supercritical essential oils may produce more therapeutic end compositions making them a great addition to the clinician’s natural toolbox.
Essential oils could therefore be defined as “mixtures of volatile aromatic compounds and select nonvolatile compounds (e.g. coumarins and fatty acids) extracted from plants by various methods that do not introduce foreign substances (e.g. solvent residue) during the extraction process.” Doing so, allows us to use plant extracts for therapeutic purposes to improve overall well-being and removes self-imposed limitations.
A consensus definition among scientists, health professionals, and the aromatherapy community will likely never be obtained. More importantly than a consensus definition, scientists and sellers of essential oils should clearly delineate what extraction method was used for the product they are evaluating or selling. So the bottom line is, the definition of essential oils depends on whether you abide by traditionally held views, definitions from published research, or a hybrid.