1. In math, consisting of three terms, as an algebraical expression.
2. In biology, relating to or consisting of three taxonomic names, denoting the genus, species, and subspecies or variety of an organism.
3. Of the names of married women (especially, in the U.S.): consisting of three elements, the given, maiden, and husband’s surname; also applied to those known by this style, whereby the maiden name is in some measure retained.
4. The name of a subspecies or variety when composed of three terms (the names of the genus, species, and subspecies or variety).