'Trimester' definitions:

Definition of 'trimester'

From: WordNet
noun
A period of three months; especially one of the three three-month periods into which human pregnancy is divided
noun
One of three divisions of an academic year

Definition of 'Trimester'

From: GCIDE
  • 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]