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]