tri·mes·ter \(ˈ)trī|mestə(r)\noun (-s) Etymology: French trimestre, from Latin trimestris of three months, from tri- + mensis month — more at moon 1.: a period of three or about three months 2.: one of three terms into which the academic year is divided at some educational institutions • tri·mes·tral\-strəl\adjective • tri·mes·tri·al\-rēəl\adjective