Webster's English Dictionary

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semester

se.mes.ter or se.mes.tri.al \s*-'mes-t*r\ \-tr*l\ \-tre--*l\ n [G, fr. L 
   semestris half-yearly, fr. sex six + mensis month -m more at SIX, MOON 1: a 
   period of six months  2: either of the two usu. 18-week periods of 
   instruction into which an acad emic year is usu. divided - se.mes.tra.l aj