The Collaborative International Dictionary of English v.0.48:

Castle \Cas"tle\, v. i. [imp. & p. p. Castled; p. pr. & vb. n.
   Castling.] (Chess)
   To move the castle to the square next to king, and then the
   king around the castle to the square next beyond it, for the
   purpose of covering the king.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Castling \Cast"ling\, n.
   That which is cast or brought forth prematurely; an abortion.
   --Sir T. Browne.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Castling \Cas"tling\, n. (Chess)
   A compound move of the king and castle. See Castle, v. i.
   [1913 Webster]

WordNet (r) 3.0 (2006):

castling
    n 1: interchanging the positions of the king and a rook [syn:
         castle, castling]