A fortnight is fourteen nights, or two weeks. This word isn't used much in American English, but you'll come across it if you travel to England — or read a lot of old British novels.
The word fortnight is still in use in Great Britain and some former British colonies. It comes from the Old English, and is literally a shortened form of fourteen nights. People sometimes use it when they're discussing their vacations or their pay schedules. In the United States, however, people typically just say "two weeks."