What is the meaning of Castling?

An abortion, or a premature birth.

The second or third swarm of bees which leaves a hive in a season.

A miniature cast or mould.

One that is cast.

A move in which the king moves two squares towards a rook, and the rook moves to the other side of the king; the action of the verb to castle.

The act of constructing a defense structure in Japanese chess in which the king (玉) is positioned in a certain way so that it is protected by pawns (歩) and silver general(s) (銀) and/or gold general(s) (金) often with an additional knight (桂) and lance (香車).

present participle and gerund of castle

Source: wiktionary.org