A pure mate is a special type of checkmate, mostly used in chess compositions for its aesthetic value. A mate is called “pure” when the opponent’s king is being checked by only one piece, and each square around the king is being attacked by only one piece, or if the square is occupied by a friendly piece. Sometimes, if the player is in a special situation where they are being attacked by two pieces (a double check), this can also be considered a pure mate.
Although their occurrence is incidental in real games, several famous games have concluded with a pure mate – for instance, the Peruvian Immortal or the Immortal Game.
Have a look at another classic example from the 19th century: