Move the king two squares toward a rook and in the same move the rook to the square next past the king WordNet 3.0 & Open English WordNet

To move the king 2 squares right or left and, in the same turn, the nearest rook to the far side of the king. The move now has special rules: the king cannot be in, go through, or end in check; the squares between the king and rook must be vacant; and neither piece may have been moved before castling. Wiktionary