The Collaborative International Dictionary of English v.0.48:

Byelorussian \Byelorussian\ n.
   1. a native or inhabitant of Byelorussia.

   Syn: Belorussian, White Russian.
        [WordNet 1.5]

   2. the language spoken in Belarus.

   Syn: Belarusian.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Byelorussian
    n 1: a native or inhabitant of Byelorussia [syn: Byelorussian,
         Belorussian, White Russian]
    2: the Slavic language spoken in Belarus [syn: Belarusian,
       Byelorussian, White Russian]