Webster's Revised Unabridged Dictionary · 1913

Castilian

Castilian , noun

[Sp. castellano, from Castila, New Latin 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.
2.
The Spanish language as spoken in Castile.