I. trī-ˈnō-mē-əl noun
Etymology: tri- + -nomial (as in binomial )
Date: 1674
1. : a polynomial of three terms
2. : a biological taxonomic name of three terms of which the first designates the genus, the second the species, and the third the subspecies or variety
II. adjective
Date: circa 1704
1. : consisting of three mathematical terms
2. : of, relating to, or being a biological trinomial