The Collaborative International Dictionary of English v.0.48:

Byelorussia \Byelorussia\ n.
   a European country east of Poland, formerly a part of the
   Soviet Union. After the breakup of the Soviet Union the name
   was changed to Belarus.

   Syn: Belarus, Belorussia, White Russia.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Byelorussia
    n 1: a landlocked republic in eastern Europe; formerly a
         European soviet [syn: Belarus, Republic of Belarus,
         Byelarus, Byelorussia, Belorussia, White Russia]