sudoku or su doku (sōōdō´kōō) [Jap.,=single number], a number puzzle consisting, in its classic form, of a square divided into nine squares, with each smaller square divided into nine boxes, thus forming an overall grid of 81 boxes. To solve the puzzle, a single number (from 1 to 9) must be entered in each blank box so that each number is used only once in each smaller square and only once in each row and column in the larger square. A number of the boxes are always filled in by the maker of the puzzle, providing both clues and limitations to where the solver may place numbers, and the degree of difficulty of the puzzle is determined by which numbers the puzzle maker chooses to reveal as clues. Variations include the use of larger or smaller squares than the 9-box-by-9-box classic grid, irregular divisions of the overall grid, and additional restrictions on the placement of the numbers.

The modern origins of sudoku appear to lie with Dell Puzzle Magazines, which has published nine-square "Number Place" puzzles since the 1970s. Such puzzles are based on, but more complicated than, the Latin squares described by the 18th-century Swiss mathematician Euler. In 1984 Nikoli publishers began publishing sudoku puzzles in Japan. Nikoli limited the number of boxes filled in as clues to less than 30, and required that the clues form a symmetrical pattern. In this form sudoku became popular in Japan and by 2005 common in the United States and other countries.