Commonly used in the UK, a fortnight is the equivalent of two weeks or really as 14 days, from Old English FOURteen NIGHTs. (Not to be confused with four score, which is 80.)

If you've not heard it used before, you wouldn't be the first. This editor had to look it up the first time the midwife said it.

“I suppose in about fortnight we shall be told that he has been seen in San Francisco. It is an odd thing, but everyone who disappears is said to be seen at San Francisco. It must be a delightful city, and possess all the attractions of the next world.”

― Oscar Wilde, The Picture of Dorian Gray