Chess, sometimes also known as international chess, is a board game and mental-skill game for two players. It is played on a square board of eight rows (called ranks) and eight columns (called files), giving 64 squares of alternating colour, light and dark, with each player having a light square at the bottom right when facing the board. Each player begins the game with 16 pieces that each move and capture other pieces on the board by defined directions and in some instances, limited range: eight pawns, two knights, two bishops, two rooks, one queen and one king. One player (who is always first to move) controls the white pieces; the other player controls the black pieces.

In chess, when a player's king is directly attacked by one or more of the opponent's pieces, the player is said to be in 'check'. When in check, only moves that can evade check or block check are permitted. The object of the game is to checkmate the opponent; this occurs when the opponent's king is in check, and no move can be made that would escape from check.

Chess is not a game of chance; it is based solely on tactics and strategy. Nevertheless, the game is so complex that not even the best players can consider all contingencies: although only 64 squares and 32 pieces are on the board, the number of possible games that can be played far exceeds the number of atoms in the universe (see Shannon number).

Chess is one of the world's most popular games; it has been described not only as a game but also as an art or science. Chess is sometimes seen as an abstract wargame; as a "mental martial art", and teaching chess has been advocated as a way of enhancing mental prowess. Chess is played both recreationally and competitively in clubs, tournaments, online, and by mail (correspondence chess). Many variants and relatives of chess are played throughout the world. The most popular, in descending order by number of players, are Xiangqi (in China), Shogi (in Japan), and Janggi (in Korea).