Biannual or Biennial?

English word “perennial” means “lasting a very long time”. This is easy to understand by dividing it into “per” and “ennial”, meaning “through” and “years”, respectively.

The “-ennial” or “-annual” part comes from the Latin word “annus” that means “year”. There are some common English words derived from annus: annual, anniversary, millennial, annals, etc.

You can use the word “${X}ennial” with a numeral prefix X to mean “lasting for X years” or “taking place once in every X years”, e.g., biennial, triennial, quadrennial, and so on.

One thing to note here is that there are two commonly accepted words: “biannual” and “biennial”. Since “-ennial” denotes “years”, biennial only means “once in every two years”. However, “biannual” is an ambiguous word that can mean both “once in every two years” and “twice in a year” (cf. biweekly).