Fortnight is a commonly used word in Britain and many Commonwealth countries such as India and Australia where many wages, salaries and most social security benefits are paid on a fortnightly basis. The word is rarely used in the United States, but is used regionally in Canada. Payroll systems may use the term biweekly in reference to pay periods every two weeks. The terms fortnightly and biweekly are often mistakenly conflated with semimonthly.

In many languages, there is no single word for a two-week period and the equivalents of "two weeks" or "fourteen days" have to be used. In Spanish, Italian, French and in Portuguese, there are the terms quincena (or quince días), quindici giorni, quinzaine and quinzena, all meaning "fifteen days". Similarly, in Greek, the term dekapenthimero meaning "fifteen days" is also used. In Arabic the short term "أسبوعان" (two weeks) is commonly used and the longer term "خمسة عشر يوماً" (fifteen days) is used to a lesser extent. In Hebrew the term "שבועיים" (two weeks) is commonly used.

In Welsh the term pythefnos, meaning "fifteen nights", is used instead. This is in keeping with the Welsh term for a week, which is wythnos ("eight nights")

The fortnight is the base unit of time in the humorous FFF System of units.

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