The Collaborative International Dictionary of English v.0.48:

Semiweekly \Sem`i*week"ly\, a.
   Coming, or made, or done, once every half week; as, a
   semiweekly newspaper; a semiweekly trip. -- n. That which
   comes or happens once every half week, esp. a semiweekly
   periodical. -- adv. At intervals of half a week each.
   [1913 Webster]

WordNet (r) 3.0 (2006):

semiweekly
    adv 1: twice a week; "he called home semiweekly" [syn:
           semiweekly, biweekly]
    adj 1: occurring twice a week [syn: semiweekly, biweekly]
    n 1: a periodical that is published twice each week (or 104
         issues per year)

Moby Thesaurus II by Grady Ward, 1.0:

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