Short answer: The word fortnight derives from an Old English word and is defined as a period of fourteen nights.
Unfortunately, there isn’t much to write about here. The etymology of the word fortnight is rather bland. It’s short for “fourteen nights.” More specifically, it comes from the Old English “fēowertīene niht,” which has the same meaning. In the U.S., we generally use the term biweekly instead of fortnight.
In astronomy, a fortnight is half of a synodic month (14.8 days), which is equivalent to the mean time between a full moon and a new moon.