trinomial

from The Collaborative International Dictionary of English v.0.48

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}.
   [1913 Webster]

from The Collaborative International Dictionary of English v.0.48

Trinomial \Tri*no"mi*al\, a.
   1. (Math.) Consisting of three terms; of or pertaining to
      trinomials; as, a trinomial root.
      [1913 Webster]

   2. (Nat. Hist.) Consisting of, or involving the use of, three
      terms; as, a trinomial systematic name specifying the
      genus, species, and variety.
      [Webster 1913 Suppl.]