Volatile oils are defined as oily liquids, which are entirely or almost entirely volatile without decomposition. They are also called essential oils as they are said to represent the essence of the odor constituents of the plants from which they are obtained. Volatile oils have been used in medicine, perfumery, and for flavoring since ancient times. Today they are used industrially as flavors in food, confectionary, and in the spice, perfume, and cosmetic trade. They are of considerable importance to the pharmaceutical industry as flavors and perfumes, and also due to their antiseptic, anesthetic, carminative, stimulant, insect repellant properties. Volatile oil components are also used as starting materials for the synthesis of other compounds (eugenol for vanillin).

Recent interest in aromatherapy has revived interest in essential oils due to their claimed curative effects.

Depending on the plant family, volatile oils are secreted in oil cells, in secretion ducts, or in glandular hairs of plants. They may be formed directly by the protoplasm, by decomposition of the resinogenous layer of the cell wall, or by the hydrolysis of certain glycosides. Volatile oils are present in every plant part, namely flowers, seeds, leaves, roots, bark, etc. In plants, they may act as insect repellants or insect attractants and are frequently associated with gums and resins.

The constituents of volatile oils are called terpenes referring to their biogenetic origin as they are formed from the head to tail condensation of isoprene units. The chemical nature of the volatile oil is dependent on the nature of the terpene constituents it contains. Terpenes are diverse as a chemical class and volatile oils in general are rich in mono and sesquiterpenes. These occur as mixtures of hydrocarbons and oxygenated compounds derived from these hydrocarbons. In some oils like turpentine oil, the hydrocarbons predominate and oils such as that of clove abound in oxygenated derivatives. In the case of many oils, the oxygenated terpenes determine the odor, taste, and even therapeutic properties of the oil and the hydrocarbons merely act as diluents in the oil.

Monoterpenes are biogenetically formed from 2 isoprene units and are categorized as acyclic, monocyclic, and bicyclic terpenes.

• Acyclic monoterpenes are considered the most important isolates in perfumery and occur as optical or geometrical isomers, which may be found together in the same oil or in different oils. For example, citral, geraniol, linalol, citronellol, and nerol are oxygenated; myrcene and ocimene are hydrocarbons.
• Monocyclic monoterpenes also occur as optical and structural isomers. For example, limonene is a widely distributed monocyclic hydrocarbon and terpineol, cineole, menthol, and carvone are oxygenated.
• In bicyclic monoterpenes, the first ring has six members and common to all, while the size of the second ring varies. These also occur as isomers with both forms occurring in the same or different plant sources. For example, pinene is a hydrocarbon, and camphor, borneol, and eugenol are oxygenated.

Sesquiterpenes are formed from three isoprene units and they constitute the higher boiling fraction of essential oils. On the basis of the number of rings in the structure, they are divided into four groups:

• Acyclic: For example, farnesene is a hydrocarbon; farnesol and nerolidol are oxygenated.
• Monocyclic: For example, zingiberene and bisabolene are hydrocarbons; gingerol is oxygenated.
• Bicyclic: For example, cadinene and caryophyllene are hydrocarbons; santalol is oxygenated.
• Tricyclic: For example, longifolene and copaene are hydrocarbons; patchoulol is oxygenated.