The Collaborative International Dictionary of English v.0.48:

Semiannual \Sem`i*an"nu*al\, a.
   Half-yearly.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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