Any of a large class of compounds derived from multiples of the unsaturated hydrocarbon isoprene. They include the terpenes, which are all hydrocarbons. Terpenoids based on one, two, three, or four *isoprene subunits
are called hemiterpenoids (e.g. apiose), monoterpenoids (e.g. camphor), sesquiterpenoids (e.g. zingiberine), and diterpenoids (e.g. phytol) respectively. The terpenoids are a very diverse class of compounds. They may be cyclic, e.g. cannabidiol, or acyclic, e.g. phytol, a precursor of chlorophyll. Many give the characteristic odour or flavour to a plant oil, e.g. limonene (lemon oil), pinene (pine oil), and menthol (mint oil). The fat-soluble vitamins A, E, and Κ are terpenoids as are the ubiquinones and plastoquinones and the growth substances gibberellic acid and abscisic acid. The carotenoids (tetraterpenoids) and sporopollenin, gutta-percha, and natural rubber (polyterpenoids) are further examples of this group. Terpenoid distribution is considered potentially valuable in taxonomic work. For example, various changes have been suggested at the tribe level in the Compositae following work on the occurrence of the bitter-tasting sesquiterpene lactones. In the genus Pinus, the distribution of turpentines has been used to justify placement of P. jeffreyi in the Macrocarpae. Traditionally it was placed in the Australes on the basis of its resemblance to P. ponderosa, a typical member of the Australes.