A fortnight is a unit of time equal to fourteen days (two weeks). The word derives from the Old English: fēowertyne niht, meaning "fourteen nights". Fortnight and fortnightly are commonly used words in Britain, Ireland and many Commonwealth countries such as Australia, India, New Zealand, and Pakistan, where many wages and salaries and most social security benefits are paid on a fortnightly basis. The word is rarely used in North America, except regionally in Canada and in insular traditional communities (e.g. Amish) in the United States. American and Canadian payroll systems may use the term biweekly in reference to pay periods every two weeks. Neither term should be confused with semimonthly (in one year there are 26 fortnightly or biweekly versus 24 semimonthly pay periods).
In astronomy, a fortnight is the mean (average) time between a full moon and a new moon (and vice versa) or half a synodic month. This is equal to 14.77 days. For more information see eclipse cycle.  In the Hindu calendar this period is called a Paksha (also Paksa) and consists of 15 Tithi.
In other languages 
In many languages, there is no single word for a two-week period and the equivalents of "two weeks" or "14 days" have to be used. In many romance languages, there are the terms quincena (or quince días) in Spanish which in payroll usage means the same as "half month", the proper name for a Fortnight is a catorcena meaning 14 days. In Galician, quindicina Italian, quinzena (Portuguese and Catalan), and quinzaine (French), all meaning "15 days" (because of inclusive counting); Italian also has the word "bisettimanale", meaning biweekly. Similarly, in Greek, the term δεκαπενθήμερο (dekapenthímero), meaning "15 days", is also used. However in Hebrew, the single-word (shvu′ayim) שבועים means exactly "two weeks". The Hindu calendar uses the Sanskrit word "paksha" to mean one half of a lunar month, which is between 14 and 15 solar days. In Welsh, the term pythefnos, meaning "15 nights", is used instead. This is in keeping with the Welsh term for a week, which is wythnos ("eight nights"). In Irish, the term "coicís" has the same meaning as fortnight.
See also