any of a class of hydrocarbons occurring widely in plants and animals and empirically regarded as built up from isoprene, a hydrocarbon consisting of five carbon atoms attached to eight hydrogen atoms (C₅H₈). The term is often extended to the terpenoids, which are oxygenated derivatives of these hydrocarbons.

Biological formation of the terpenes occurs by the combination of two molecules of acetic acid to give mevalonic acid (C₆H₁₂O₄) and conversion of the latter to isopentenyl pyrophosphate, which contains the five-carbon isoprene skeleton. Further transformations of the isopentenyl compound yield the true terpenes and the terpenoids.

The true terpenes are usually grouped according to the number of isoprene (C₅H₈) units in the molecule: monoterpenes (C₁₀H₁₆) contain two such units; sesquiterpenes (C₁₅H₂₄), three; diterpenes (C₂₀H₃₂), four; triterpenes (C₃₀H₄₈), six; and tetraterpenes (C₄₀H₆₄), eight. Rubber and gutta-percha are polyterpenes in which 1,000–5,000 isoprene units are joined in a long chain. Monoterpenes, sesquiterpenes, and diterpenes are abundant in the essential oils of plants: turpentine contains several monoterpenes, and the rosin acids are diterpenes. Vitamin A is another important diterpene. The triterpene squalene, obtainable from shark-liver oil, may be converted to cholesterol and many other steroids. The carotenoid pigments are the best known tetraterpenes.