: any of a class of volatile oils that give plants their characteristic odors and are used especially in perfumes and flavorings, and for aromatherapy — compare fixed oil
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: any of a large class of volatile odoriferous oils of vegetable origin that give plants their characteristic odors and often other 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 aldehydes or esters), and that are used often in the form of essences in perfumes, flavorings, and pharmaceutical preparations