In zoology and botany:Consisting of three terms, as the technical name of a subspecies; trionymal: thus, the name Certhia familiaris americana is trinomial. See binomial, polynomial.Using or admitting trinomial or trionymal names in certain cases: as, the trinomial system of nomenclature. Also trinominal.In algebra, consisting of three terms connected by either of the signs + and—: thus, a + b + c, or x—2 xy + y is a trinomial quantity.n. A technical name consisting of three words, of which the first is the name of the genus, the second that of the species, and the third that of a geographical race, subspecies, or variety; a trionym.n. In algebra, a trinomial expression. See I., 2.