Terpenes are volatile constituents of plant resins and essential oils

Terpenes are volatile constituents of plant resins and essential oils... 

The term terpene is used to describe a compound, which is a constituent of an essential oil containing carbon and hydrogen or carbon atoms, hydrogen, and oxygen atoms, and is not aromatic in character [24, 25]. This definition is usually extended to include other compounds called terpenoids, which are not of natural occurrence but are very closely related to the natural terpenes. Most terpenes, which include terpenoids, are invariably hydrocarbons, alcohols, aldehydes, ketones, or oxides, and they may be solids or liquids. Terpene hydrocarbons are usually liquids, while terpenes of higher molecular weights, mostly obtained from the natural gums and resins of plants and trees, are not steam volatile. 

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