A fortnight is a unit of time equal to 14 days (2 weeks). The word derives from the Old English: fēowertyne niht, meaning "fourteen nights".
Some wages, salaries, and social security benefits are paid on a fortnightly basis; however in North America it is far more common to use the term biweekly. Neither of these terms should be confused with semimonthly, which divides a year into exactly 24 periods (12 months × 2), instead of the 26 (≈52 weeks ÷ 2) of fortnightly/biweekly.
For more information see eclipse cycle.
In astronomy, a fortnight is half a synodic month, the mean time between a full moon and a new moon (and vice versa). This is equal to 14.77 days.
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.
Celtic languages: in Welsh, the term pythefnos, meaning "15 nights", is used. This is in keeping with the Welsh term for a week, which is wythnos ("eight nights"). In Irish, the term is coicís.
Similarly in Greek the term δεκαπενθήμερο (dekapenthímero), meaning "15 days", is used.
The Hindu calendar uses the Sanskrit word "paksha", meaning one half of a lunar month, which is between 14 and 15 solar days.
In Romance languages there are the terms quincena (or quince días) in Galician and Spanish, quinzena in Catalan and Portuguese, quindicina in Italian, quinze jours or quinzaine in French, and chenzinǎ in Romanian, all meaning "a grouping of 15"; there are also the terms bisettimanale in Italian, bisemanal in Spanish, bissemanal in Portuguese, bisetmanal in Catalan, bihebdomadaire in French, and bisǎptǎmânal in Romanian, that literally mean "biweekly".
Semitic languages have a special "doubling suffix". When added at the end of the word for "week" it changes the meaning to "two weeks". In Hebrew, the single-word שבועיים (shvu′ayim) means exactly "two weeks". Also in Arabic, by adding the common dual suffix to the word for "week", أسبوع, the form أسبوعين (usbu′ayn), meaning "two weeks", is formed.
Slavic languages: in Czech the terms čtrnáctidenní and dvoutýdenní have the same meaning as "fortnight".
