Fortnightly means happening once every two weeks, or appearing every 14 days. If your piano teacher schedules fortnightly lessons, you'll have class one week, skip the next week, then return for another lesson. A magazine published fortnightly arrives twice a month, roughly every other week.

While Americans usually say “every two weeks” or “biweekly,” people in Britain, Australia, and other English-speaking countries commonly use fortnightly in everyday conversation.

You might see fortnightly in older books or formal writing: “The committee meets fortnightly” or “She receives a fortnightly allowance.” As a noun, a fortnightly can also mean a publication that comes out every two weeks, though this usage is less common today.

Be careful with “biweekly,” which can confusingly mean either twice a week or once every two weeks, depending on context. Fortnightly is clearer because it only means one thing: every 14 days.