(n.) A term or period of three months.

abundant year academic year annum bissextile year calendar month calendar year century clover common year day decade decennary decennium defective year deuce-ace fiscal year fortnight hour leap year leash lunar month lunar year lunation luster lustrum man-hour microsecond millennium millisecond minute moment month moon quarter quinquennium regular year second semester session set of three shamrock sidereal year solar year sun tercet term ternary ternion terzetto three threesome tierce trefoil trey triad trialogue triangle tricorn trident triennium trihedron trilogy trine trinity trinomial trio triphthong triple crown triple threat triplet triplopy tripod triptych trireme triseme triskelion trisul triumvirate triunity trivet troika twelvemonth week weekday year