A hyponym (in Greek: υπονύμιον, literally meaning 'few names') is a word that is conceptually included within the definition of another word, as scarlet, vermilion, carmine and crimson are all hyponyms of red. Although closely related in meaning to synonym, they are two different semantic categories. Tulip is a hyponym of flower, for instance, but not a synonym. Hyponymy is the semantic relation in which one word is the hyponym of another.
In his classic formulation, the linguist C.E. Bazell stated, "There is a relation of hyponymy when one word may invariably be replaced by a second word, but not vice-versa, without change of meaning."
See also