The term “essential oil” was coined by 16th Century alchemist Paracelsus who thought that these substances contained the quintessence of plants. Centuries earlier, Greek philosophers had proposed that matter appears in four forms (air, earth, fire, and water), and Aristotle had proposed a fifth form—in Latin, the quinta essentia—which would represent the essence of things. To Paracelsus, the oils represented the most concentrated form of the individual characteristics of the plant and that is its essence.
An essential oil is a volatile material distilled or expressed from a plant. They are complex mixtures of volatile organic compounds (VOCs) produced by living organisms and isolated by physical means (pressing and distillation) from a whole plant or plant part of known taxonomic origin” (Franz and Novak, 2010.) Therefore, essential oils are not produced by plants. Instead, they are distilled, expressed or extracted by these physical methods.
Steam distillation is the easiest and most common means of isolating these VOCs. In steam distillation, plant material is exposed to steam, whose heat causes these compounds to evaporate. Subsequent cooling of the hot vapors causes condensation of both water vapor and the volatile organic compounds