The Collaborative International Dictionary of English v.0.48:

Castilian \Cas*til"ian\, n. [Sp. castellano, from Castila, NL.
   Castilia, Castella. Castile, which received its name from the
   castles erected on the frontiers as a barrier against the
   Moors.]
   1. An inhabitant or native of Castile, in Spain.
      [1913 Webster]

   2. The Spanish language as spoken in Castile.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Castilian
    n 1: the Spanish language as spoken in Castile