Vitamins occur in a variety of related forms known as vitamers. A vitamer (/ˈvaɪtəmər/) of a particular vitamin is one of several related compounds that exhibit biological activity against a specific vitamin deficiency. A set of vitamers with related biological activity are grouped together by a general name, or generic descriptor, that refers to similar compounds with the same vitamin function. For example, vitamin A is the generic descriptor for the class of vitamin A vitamers which include retinol, retinal, retinoic acid, and provitamin carotenoids such as beta-carotene among others.