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

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "trimester":
   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