When plant matter (leaves, roots, flowers, etc.) is subjected to steam distillation, an oily liquid called essential oil is obtained. An essential oil is a mixture of terpenes and/or terpenoids. It is a substance, possessing the aroma, nutrients and characteristic properties of the plant, that is required for normal functioning but cannot be synthesized by the body and therefore must be included in the diet. The resulting oil mixture of organic compounds was thought to be the essence (the most important ingredients) of the plant, hence the word essential oils.
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