1. The act of moving the king and rook simultaneously. This is the only time in the game where two pieces can be moved in the same turn. Castling consists of moving the king two squares either right or left, and placing the rook on the square beside the king closest to the centre. There must be no pieces between king and rook, neither piece may have already moved, and the King may not move out of Check, over it, or into it. Castling is usually worthwhile because it moves the King to a safer position in the wings behind pawns, and the rook to a more powerful position in the centre of the board at the same time.
2. Unsophisticated term for Rook.