Source : Webster's Revised Unabridged Dictionary (1913)

Fortnight \Fort"night`\ (?; in U.S. often ?; 277), n. [Contr.
   fr. fourteen nights, our ancestors reckoning time by nights
   and winters; so, also, seven nights, sennight, a week.]
   The space of fourteen days; two weeks.

Source : WordNet®

fortnight
     n : a period of fourteen consecutive days; "most major tennis
         tournaments last a fortnight" [syn: {two weeks}]