The Collaborative International Dictionary of English v.0.48:

Trichotomy \Tri*chot"o*my\, n. [Gr. tri`cha threefold, in three
   parts + te`mnein to cut or divide: cf. F. trichotomie.]
   Division into three parts.
   [1913 Webster]

WordNet (r) 3.0 (2006):

trichotomy
    n 1: being threefold; a classification into three parts or
         subclasses