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]