Webster's Revised Unabridged Dictionary · 1913

Trinomial

Trinomial , noun

[Prefix tri- + -nomial as in binomial: compare French trinôme.]

(Mathematics) A quantity consisting of three terms, connected by the sign + or -; as, x + y + z, or ax + 2b - c2.

Trinomial , adjective

1.
(Mathematics) Consisting of three terms; of or pertaining to trinomials; as, a trinomial root.
2.
(Nat. Hist.) Consisting of, or involving the use of, three terms; as, a trinomial systematic name specifying the genus, species, and variety.