One half of a school year, or two quarters, or a third of a year depending on the school. Where I'm from, in primary schools anyway, each quarter is nine weeks, making each semester eighteen. At the university I went to, it was sixteen weeks per semester. It's not really that long, but it seems like it. Now that I'm gainfully employed, time goes too fast instead.

Se*mes"ter (?), 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.

© Webster 1913.