A fortnight is two weeks, from the Old English for fourteen nights. It is everyday vocabulary in the UK, Ireland, Australia and New Zealand, where pay cycles, sprints and reviews run fortnightly, and almost unknown in American offices, where the same interval is just “every two weeks” or “biweekly”.
Biweekly is its own trap, meaning either twice a week or every two weeks depending on the speaker. Global teams do well to standardise: fortnightly where audiences understand it, “every two weeks” where they might not, and biweekly nowhere.