Essential oils are composed of hydrophobic volatile organic compounds, usually in liquid form, obtained from plant material. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the oil of the plant from which they were extracted, such as oil of clove. An essential oil is essential in the sense that the oil contains the fragrance or essence of the plant. The term "essential" used here does not indicate that it is nutritionally required in the diet, as with essential amino acid or essential fatty acid.

Essential oils, according to ISO, are produced either by steam distillation, by mechanical manipulation of the rind of the fruits of the genus Citrus (also called cold pressing), or by dry distillation. Other methods, such as solvent extraction, are not accepted and, in fact, result in different perfumed products (pommades, concretes, and absolutes). They are used in perfumes, cosmetics, soaps, air fresheners and other products, for flavoring food and drink, and for adding scents to household cleaning products.

Essential oils are often used for aromatherapy. Improper use of essential oils may cause harm including allergic reactions, inflammation and skin irritation. Children may be particularly susceptible to the toxic effects of improper use. Essential oils can be toxic if ingested or absorbed through the skin.