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1. A period or term of three months.
2. One of three terms into which an academic year is divided in some universities and colleges.
[French trimestre, from Latin trimēstris, trimēnstris, of three months : tri-, tri- + mēnsis, month; see mē- in Indo-European roots.]
tri·mes′tral (-trəl), tri·mes′tri·al (-trē-əl) adj.