The Collaborative International Dictionary of English v.0.48:

Semimonthly \Sem`i*month"ly\a.
   Coming or made twice in a month; as, semimonthly magazine; a
   semimonthly payment. -- n. Something done or made every half
   month; esp., a semimonthly periodical. -- adv. In a
   semimonthly manner; at intervals of half a month.
   [1913 Webster]

WordNet (r) 3.0 (2006):

semimonthly
    adv 1: twice a month; "salaries are paid semimonthly" [syn:
           semimonthly, bimonthly]
    adj 1: occurring twice a month; "a semimonthly publication"
           [syn: semimonthly, bimonthly]
    n 1: a periodical that is published twice each month (or 24
         issues per year)

Moby Thesaurus II by Grady Ward, 1.0:

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