a specified period of five years
‘From First to Fifth Year, students prepared to take their O.W.L.s, and after that quinquennium of schooling the time finally came.’
quinquennia, quinquenniums, quinquennial
Useful word? Funny to say, anyhow.
Have a nice quinquennium! (Well, a quinquennium is a specified period of five years, but I’m allowed to set a bad example, aren’t I?)