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Merriam-Webster's Unabridged Dictionary defines essential oil as follows:
[A]ny of a large class of volatile odoriferous oils of vegetable origin that impart to plants odor and often other characteristic properties, that are obtained from various parts of the plants (as flowers, leaves, or bark) by steam distillation, expression, or extraction, that are usually mixtures of compounds (as terpenoids, aldehydes, or esters), and that are used often in the form of essences in perfumes, flavoring materials, and pharmaceutical preparations -- called also ethereal oil, volatile oil.
The U.S. National Cancer Institute, commenting on essential oils because of its use as 'supportive therapy' by cancer patients to improve aspects of well-being, defines essential oils as follows:
The scented liquid taken from certain plants using steam or pressure. Essential oils contain the natural chemicals that give the plant its “essence” (specific odor and flavor). Essential oils are used in perfumes, food flavorings, medicine, and aromatherapy.
Hence, the qualifier, essential, does not have the same sense of 'required' for health as does "essential amino acids" or "essential fatty acids", but only the sense of 'required' as required to give a plant it characteristic, or specific, odor or flavor.
Familiar examples of essential oils include those extracted from species of citrus, lavender, seeds of celery and pumpkin, eucalyptus, and rosewood. Known essential oils number in the hundreds.
In addition to their use in perfumes and flavorings, essential oils have been used in complementary and alternative medicine as antimicrobials. Microbiologists Hammer and colleagues studied the in vitro antimicrobial activity of 52 essential oils, reporting as follows:
In summary, this study confirms that many essential oils and plant extracts possess in vitro antibacterial and antifungal activity. However, if plant oils and extracts are to be used for food preservation or medicinal purposes, issues of safety and toxicity will need to be addressed.
- See also: Distillation
Some common essential oils, such as lavender, peppermint, tea-tree and eucalyptus, are distilled. Raw plant material, consisting of the flowers, leaves, wood, bark, roots, seeds, or peel, is put into an alembic (distillation apparatus) over water. As the water is heated the steam passes through the plant material, vaporizing the volatile compounds. The vapors flow through a coil where they condense back to liquid, which is then collected in the receiving vessel.
Aromatherapy is a form of complementary and alternative medicine, in which healing effects are ascribed to the aromatic compounds in essential oils and other plant extracts. Many common essential oils have medicinal properties that have been applied in folk medicine since ancient times and are still widely used today. For example, many essential oils have antiseptic properties, though some are stronger than others.. In addition, many are claimed to have an uplifting effect on the mind, though different essential oils have different properties. The claims are supported in some studies and unconfirmed in others.
The odoriferous quality of essential oils lends itself to the practice called aromatherapy, wherein the inhalation of the odors of an oil may yield positive health effects of various types. In some respects, assessing its benefits are more difficult than with other forms of complementary medicine: whether or not the essential oils actually give benefit, they smell much better than the average hospital.
Some of the clinical trials that are more suggestive of benefits have two characteristics:
- Aromatherapy is complementary to medical therapy
- Aromatherapy is complementary to other complementary therapies: it may, for example, be effective in combination with massage but not on its own.
Skin application, not necessarily massage, have been tested as more effective than inhalation alone. Another study suggested that applying the oils to the skin may be more effective than inhalation.  Massage may be of greater benefit than either inhalation or simple application. 
Training in aromatherapy does not convey additional diagnostic skill. 
The National Cancer Institute (NCI), linked through the National Center for Complementary and Alternative Medicine (NCCAAM), provides the following overview of aromatherapy for health professional:
- Aromatherapy is the therapeutic use of essential oils (also known as volatile oils) from plants (flowers, herbs, or trees) for the improvement of physical, emotional, and spiritual well-being.
- Aromatherapy is used by patients with cancer primarily as supportive care for general well-being.
- Aromatherapy is used with other complementary treatments (e.g., massage and acupuncture) as well as standard treatment.
- Essential oils are volatile liquid substances extracted from aromatic plant material by steam distillation or mechanical expression; oils produced with the aid of chemical solvents are not considered true essential oils.
- Essential oils are available in the United States for inhalation and topical treatment. Topical treatments are generally used in diluted forms.
- Aromatherapy is not widely administered via ingestion.
- The effects of aromatherapy are theorized to result from the effect of odorant molecules from essential oils on the brain’s emotional center, the limbic system. Topical application of aromatic oils may exert antibacterial, anti-inflammatory, and analgesic effects.
- Studies in animals show sedative and stimulant effects of specific essential oils as well as positive effects on behavior and the immune system. Functional imaging studies in humans support the influence of odors on the limbic system and its emotional pathways.
- Human clinical trials have investigated aromatherapy primarily in the treatment of stress and anxiety in patients with critical illnesses or in other hospitalized patients. Several clinical trials involving patients with cancer have been published.
- Aromatherapy has a relatively low toxicity profile when administered by inhalation or diluted topical application.
- Aromatherapy products do not need approval by the U.S. Food and Drug Administration because there is no claim for treatment of specific diseases.
- Repeated exposure to lavender and tea tree oils by topical administration has been associated with reversible prepubertal gynecomastia.
Several common essential oils, such as cinnamon and clove, can cause skin burns if applied in undiluted form. 
Clove oil is commonly used in dentistry, to reduce tooth and gum pain.
- "essential oil." Webster's Third New International Dictionary, Unabridged. Merriam-Webster, 2002. http://unabridged.merriam-webster.com (28 Sep. 2010).
- Dictionary of Cancer Terms: Essential Oils.
- Hammer KA, Carson CF, Riley TV. (1999) Antimicrobial activity of essential oils and other plant extracts. J Appl Microbiol 86:985-90. PMID 10438227
- Seenivasan Prabuseenivasan, Manickkam Jayakumar, and Savarimuthu Ignacimuthu (November 30, 2006). "In vitro antibacterial activity of some plant essential oils". BMC Complement Altern Med. 6 (39). DOI 10.1186/1472-6882-6-39. Retrieved on 2006-12-22.
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- Hiroko Kuriyama, Satoko Watanabe, Takaaki Nakaya, Ichiro Shigemori, Masakazu Kita, Noriko Yoshida, Daiki Masaki, Toshiaki Tadai, Kotaro Ozasa, Kenji Fukui, and Jiro Imanishi (September 15, 2005). "Ambient odors of orange and lavender reduce anxiety and improve mood in a dental office". Physiol Behav 86 (1-2): 92-5. PMID 16095639.
- Caroline Hoffman (20 December 2007), "Benefits of complementary therapies", Breast Cancer Res. 9(Suppl 2): S9., DOI:10.1186/bcr1807.
- Susie M. Wilkinson, Sharon B. Love, Alex M. Westcombe, Maureen A. Gambles, Caroline C. Burgess, Anna Cargill, Teresa Young, E. Jane Maher, Amanda J. Ramirez (10 February 2007), "Effectiveness of Aromatherapy Massage in the Management of Anxiety and Depression in Patients With Cancer: A Multicenter Randomized Controlled Trial", Journal of Clinical Oncology 25 (5): 532-539, DOI:10.1200/JCO.2006.08.9987
- Snow LA, Hovanec L, Brandt J. (2004 Jun), "A controlled trial of aromatherapy for agitation in nursing home patients with dementia.", Altern Complement Med 10(3): 431-7.
- Lehrner J, Marwinski G, Lehr S, Johren P, Deecke L (June 2005). "Immunological and Psychological Benefits of Aromatherapy Massage". Evid Based Complement Alternat Med 2 (2). DOI:10.1093/ecam/neh087. Retrieved on 2006-12-24. Research Blogging.
- Select Committee appointed to consider Science and Technology, U.K. Parliament (21 November 2000), Chapter 2: Disciplines examined, Definitions of the Various CAM Therapies, Complementary and Alternative Medicine
- Ernst, E., "Adverse effects of herbal drugs in dermatology", British Journal of Dermatology 143 (5), DOI:10.1046/j.1365-2133.2000.03822.x