“Biennial” and “biannual” are easily confused: “Biennial” means every two years, and “biannual” means twice a year. “Annual” is an easy one to remember, as is “centennial.” You’re likely to say “every three or four years,” but “triennial” and “quadrennial” are pretty easy to figure out. However, denoting five years is a bit trickier. “Quinquennial” follows the same pattern, and it comes from the Latin “quinque,” meaning “five.”