Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Castling \Cas"tling\, n. (Chess)
   A compound move of the king and castle. See {Castle}, v. i.

Source : WordNet®

castling
     n : interchanging the positions of the king and a rook [syn: {castle}]