Briefly define the Playfair cipher

The Playfair cipher is a polygraphic substitution cipher that encrypts pairs of letters instead of single letters. It uses a 5x5 grid of letters (excluding J) to create digraphs and applies a set of rules to determine the ciphertext. The Playfair cipher was invented by Charles Wheatstone in the mid-19th century and was used by the British military during World War I.