An essential oil is a concentrated hydrophobic liquid containing volatile (defined as "the tendency of a substance to vaporize") aroma compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetherolea, or simply as the oil of the plant from which they were extracted, such as oil of cove. An essential oil is "essential" in the sense that it contains the "essence of" the plant's fragrance—the characteristic fragrance of the plant from which it is derived.
The term essential used here does not mean indispensable as with the terms essential amino acid or essential fatty acid which are so called since they are nutritionally required by a given living organism. In contrast to fatty oils, essential oils typically evaporate completely without leaving a stain or residue.
Essential oils are generally extracted by distillation, often by using steam. Other processes include expressions, solvent extraction, sfumatura, absolute oil extraction, resin tapping, wax embedding, and cold pressing. They are used in perfumes, cosmetics, soaps and other products, for flavoring food and drink, and for adding scents to incense and household cleaning products.
Essential oils are often used for aromatherapy. Aromatherapy may be useful to induce relaxation. Improper use of essential oils may cause harm including allergic reactions and skin irritation, and children may be particularly susceptible to the toxic effects of improper use.
Due to the worldwide usage of essential oils for centuries, it is difficult to pinpoint exactly which culture began using them first, but the first records of essential oils allegedly come from ancient India, Persia, and Egypt. Greece and Rome also engaged in the widespread trade of aromatic oils and ointments with Eastern countries. Each culture had various uses for them ranging from health treatments to spiritual practices. Their herbal preparations included incense, perfume, clothing and fabric fresheners, powders, and suppositories, ointments, scented baths, and aromatherapy massages. In many cultures, aromatic oils were believed to create a union with the gods and were held in such high regard that they were used only by a select group of privileged people, such as priests.