A fortnight is a period of two weeks, or fourteen days. If your library book is due in a fortnight, you have exactly two weeks to finish it and return it. If you're visiting your grandmother for a fortnight, you'll be staying with her for fourteen days.

Today, fortnight is used much more commonly in Britain, Australia, and other English-speaking countries than in the United States, where people usually just say “two weeks.” But you'll still encounter the word in books, especially older ones or those written by British authors. In The Hobbit, when Bilbo Baggins receives an unexpected invitation, he's told to arrive “in a fortnight's time.”

Some things naturally happen on a fortnightly schedule: a magazine might publish every fortnight, or you might visit the orthodontist once a fortnight during treatment. The word has a precise, slightly formal feel to it, which is why it often appears in schedules, contracts, and official communications.