trinomial

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Consisting of three names or terms, as a taxonomic designation.
• adjective Mathematics Consisting of three terms.
• noun A three-part taxonomic designation indicating genus, species, and subspecies or variety, such as Brassica oleracea botrytis, the cauliflower.
• noun Mathematics An algebraic expression consisting of three terms connected by plus or minus signs, such as x² + 4x -3.

from The Century Dictionary.

• In zoology and botany:
• Consisting of three terms, as the technical name of a subspecies; trionymal: thus, the name Certhia familiaris americana is trinomial. See binomial, polynomial.
• Using or admitting trinomial or trionymal names in certain cases: as, the trinomial system of nomenclature. Also trinominal.
• In algebra, consisting of three terms connected by either of the signs + and—: thus, a + b + c, or x—2 xy + y is a trinomial quantity.
• noun A technical name consisting of three words, of which the first is the name of the genus, the second that of the species, and the third that of a geographical race, subspecies, or variety; a trionym.
• noun In algebra, a trinomial expression. See I., 2.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Math.) A quantity consisting of three terms, connected by the sign + or -.
• adjective (Math.) Consisting of three terms; of or pertaining to trinomials.
• adjective (Nat. Hist.) Consisting of, or involving the use of, three terms.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Consisting of three names or parts or terms.
• noun algebra An expression consisting of 3 terms.