A fortnight is a unit of time equal to 14 days (two weeks). The word derives from the Old English term fēowertīene niht, meaning "fourteen nights" (or "fourteen days", since the Anglo-Saxons counted by nights).
Further information: Eclipse cycle
In astronomy, a lunar fortnight is half a lunar synodic month, which is equivalent to the mean period between a full moon and a new moon (and vice versa). This is equal to 14.77 days. It gives rise to a lunar fortnightly tidal constituent (see: Long-period tides).
In many languages, there is no single word for a two-week period, and the equivalent terms "two weeks", "14 days", or "15 days" (counting inclusively) have to be used.
Wythnos is a week.