A fortnight is a unit of time equal to two weeks: that is 14 days, or literally 14 nights. The term is common in the British Isles and many Commonwealth countries such as Australia where many wages, salaries and most social security benefits are paid on a fortnightly basis.
In many languages, there is no such term to describe a time span of two weeks and the equivalents of "two weeks" or "fourteen days" have to be used. Payroll systems may use the term "bi-weekly" to refer to pay periods every 2 weeks (26 per year). In Spanish, Italian, French and in Portuguese, exist the term "quincena", "quindici giorni", "quinze jours" or "quinzena", all meaning "fifteen days" which are used to roughly name a two-weeks time span.
The fortnight is the base unit of time in the FFF (Furlong/Firkin/Fortnight) System of units.

Unusual usages

It can sometimes be used to describe a unit of time equal to five months in some Dickensian literature.
In the VMS operating system, some configuration parameters are specified in microfortnights (one millionth of a fortnight, or 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.
One attoparsec per microfortnight is approximately 1.00432766 inches per second.
The speed unit of one furlong per fortnight is a barely noticeable 0.166 millimetres per second, or roughly 1 centimetre per minute.