A fortnight is a measure of time equivalent to two weeks: that is 14 days, or literally 14 nights. The term is most commonly used in the UK. It derives from the Old English feowertiene niht meaning fourteen nights.

In the VMS operating system, some configuration parameters are specified in microfortnights (one millionth of a fortnight, or approximately 1.2096 seconds). Millifortnights (about 20 minutes) and nanofortnights (1.2096 milliseconds) have also been used occasionally in computer science, usually in an attempt to be deliberately over-complex and obscure. The aim is generally to slow users down, allowing them to set parameters only after some thought.

Interestingly enough, one attoparsec per microfortnight approximately equals one inch per second. The actual figure is approximately 1.00432766 inches per second.