The Collaborative International Dictionary of English v.0.48:

Biannual \Bi*an"nu*al\, a. [Pref. bi- + annual.]
   Occurring twice a year; half-yearly; semiannual.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

biannual
    adj 1: occurring or payable twice each year [syn: semiannual,
           biannual, biyearly, half-yearly]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "biannual":
   annual, biennial, bimonthly, biweekly, catamenial, centenary,
   centennial, daily, decennial, diurnal, fortnightly, hebdomadal,
   hourly, menstrual, momentary, momently, monthly, quarterly,
   quotidian, secular, semestral, semiannual, semimonthly, semiweekly,
   semiyearly, tertian, triennial, weekly, yearly