se·mes·ter \sə̇ˈmestə(r)\noun (-s) Etymology: German, from Latin semestris half-yearly, from sex six + mensis month — more at six, moon 1.: a half a year : a period of six months 2.: either of the two periods of instruction commonly 18 weeks in length into which an academic year is usually divided — compare quarter 4b 3.: any term of instruction : session < the second semester of summer school >