Castling

Castling is a move in chess. It consists of moving one’s king two squares toward a rook on the same  rank and then moving the rook to the square that the king passed over. Castling is permitted only if neither the king nor the rook has previously moved; the squares between the king and the rook are vacant; and the king does not leave, cross over, or end up on a square attacked by an opposing piece. Castling is the only move in chess in which two pieces are moved at once.

Castling with the king's rook is known as  castling kingside or castling short, and castling with the queen's rook is known as castling queenside or castling long.

Source: https://en.wikipedia.org/wiki/Castling