Source : Webster's Revised Unabridged Dictionary (1913)

Trinomial \Tri*no"mi*al\, a. (Nat. Hist.)
   Consisting of, or involving the use of, three terms; as, a
   trinomial systematic name specifying the genus, species, and
   variety.

Trinomial \Tri*no"mi*al\, n. [Pref. tri- + -nomial as in
   binomial: cf. F. trin[^o]me.] (Math.)
   A quantity consisting of three terms, connected by the sign +
   or -; as, x + y + z, or ax + 2b - c^{2}.

Trinomial \Tri*no"mi*al\, a. (Math.)
   Consisting of three terms; of or pertaining to trinomials;
   as, a trinomial root.