NOUN (2)

1. one of two divisions of an academic year;

2. half a year; a period of 6 months;

The Collaborative International Dictionary of English v.0.48:

Semester \Se*mes"ter\ (s[-e]*m[e^]s"t[~e]r), n. [G., from L.
   semestris half-yearly; sex six + mensis a month.]
   A period of six months; especially, a term in a college or
   university which divides the year into two terms.
   [1913 Webster]

WordNet (r) 3.0 (2006):

semester
    n 1: one of two divisions of an academic year
    2: half a year; a period of 6 months