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]

Moby Thesaurus II by Grady Ward, 1.0:

95 Moby Thesaurus words for "fortnight":
   L, Sexagesima, abundant year, academic year, annum,
   bissextile year, boxcar, boxcars, calendar month, calendar year,
   century, common year, day, decade, decennary, decennium,
   defective year, dozen, duodecimo, eighty, eleven, fifteen, fifty,
   fiscal year, five and twenty, forty, four and twenty, fourscore,
   fourscore and ten, fourteen, half a hundred, hour, leap year,
   long dozen, lunar month, lunar year, lunation, luster, lustrum,
   man-hour, microsecond, millennium, millisecond, minute, moment,
   month, moon, ninety, nonagenarian, octogenarian, quarter,
   quindecennial, quindecim, quindecima, quindene, quinquennium,
   regular year, score, second, semester, septuagenarian, session,
   seventy, sexagenarian, sexagenary, sexagesimo-quarto,
   sidereal year, sixteen, sixteenmo, sixty, sixty-four, sixty-fourmo,
   solar year, sun, teens, term, thirteen, thirty-two, thirty-twomo,
   threescore, threescore and ten, trimester, twelve, twelvemo,
   twelvemonth, twenty, twenty-five, twenty-four, twenty-fourmo,
   two dozen, two weeks, twoscore, week, weekday, year