1670s, "having three names," from tri- + second element from binomial. In mathematics, "consisting of three terms" (1704).
Entries linking to trinomial
tri-
word-forming element meaning "three, having three, once every three," from Latin tres (neuter tria) or Greek treis, trias "three" (see three).
binomial n.
1550s, "an algebraic expression consisting of two terms," from Late Latin binomius "having two personal names," a hybrid from bi- "two" (see bi-) + nomius, from nomen (from PIE root *no-men- "name"). In zoology and botany, "a name consisting of two terms, generic and specific."