The Collaborative International Dictionary of English v.0.48:

Fortnightly \Fort"night`ly\, a.
   Occurring or appearing once in a fortnight; as, a fortnightly
   meeting of a club; a fortnightly magazine, or other
   publication. -- adv. Once in a fortnight; at intervals of a
   fortnight.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fortnightly
    adv 1: every two weeks; "he visited his cousins fortnightly"
           [syn: fortnightly, biweekly]
    adj 1: occurring every two weeks [syn: fortnightly,
           biweekly]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "fortnightly":
   annual, biannual, biennial, bimonthly, biweekly, catamenial,
   centenary, centennial, daily, daybook, decennial, diary, diurnal,
   ephemeris, gazette, hebdomadal, hourly, journal, magazine,
   menstrual, momentary, momently, monthly, newsmagazine, organ,
   periodical, pictorial, quarterly, quotidian, review, secular,
   semestral, semiannual, semimonthly, semiweekly, semiyearly, serial,
   slick magazine, tertian, trade magazine, triennial, weekly,
   yearbook, yearly