A fortnight is a unit of time equal to 14 days (2 weeks). The word derives from the Old English term fēowertyne niht, meaning "fourteen nights".
In North America it is usual to say biweekly.
Neither of these synonymous terms should be confused with semimonthly, since half a month is slightly longer than a fortnight, except in February of a non-leap year.
In astronomy, a lunar fortnight is half a lunar synodic month, equivalent to the mean period between a full moon and a new moon (and vice versa). This is equal to 14.77 days.
Analogs in other languages
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 or quinze dies in Catalan and quinze dias or quinzena in 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".
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