In linguistics, a hyponym is a word or phrase whose semantic field is included within that of another word, its hypernym (sometimes spelled hyperonym outside of the natural language processing community). In simpler terms, a hyponym shares a type-of relationship with its hypernym. For example, scarlet, vermilion, carmine, and crimson are all hyponyms of red (their hypernym), which is, in turn, a hyponym of colour.
Computer science often terms this relationship an "is-a" relationship. For example, the phrase Red is-a colour can be used to describe the hyponymic relationship between red and colour.