NOUN (2)

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;

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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