Source : Webster's Revised Unabridged Dictionary (1913)

Trimester \Tri*mes"ter\, n. [L. trimestris of three months; tri-
   (see {Tri-}) + mensis month: cf. F. trimestre.]
   A term or period of three months.

Source : WordNet®

trimester
     n 1: a period of three months; especially one of the three
          three-month periods into which human pregnancy is
          divided
     2: one of three divisions of an academic year