The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fortnight
    n 1: a period of fourteen consecutive days; "most major tennis
         tournaments last a fortnight" [syn: fortnight, two
         weeks]