An academic term (or simply "term") is a portion of an academic year, the time during which an educational institution holds classes. The schedules adopted vary widely. * A quarter system divides the academic year into four terms, one per season, with attendance required in three quarters per year to total 32 to 36 weeks of instruction. * A semester system divides the academic year into two terms of equal length, with attendance required in both semesters to total 32 to 36 weeks of instruction. There is often an optional summer session half as long as a full semester. * A trimester system divides the academic year into three terms of 14 to 16 weeks each, with attendance required in two trimesters per year, to total 28 to 32 weeks of instruction. The trimester evolved from the semester s