Sudoku is a logic-based number placement puzzle. The objective is to fill a 9×9 grid so that each column, each row, and each of the nine 3×3 boxes (also called blocks or regions) contains the digits from 1 to 9, only one time each (that is, exclusively). The puzzle setter provides a partially completed grid.

The name Sudoku means "single digits"[4][5][6][7][8][9]. The name is a trademark of puzzle publisher Nikoli Co. Ltd. in Japan[10]. The word sudoku derives from the Japanese phrase "Sūji wa dokushin ni kagiru," meaning "the numbers must be single," or "the numbers must occur only once." Other Japanese publishers refer to the puzzle as Number Place, the original U.S. title, or as "Nanpure" for short. Some publishers spell the title as "Su Doku". The numerals in Sudoku puzzles are used for convenience; arithmetic relationships between numerals are irrelevant. Any set of distinct symbols will do; letters, shapes, or colours may be used without altering the rules. In fact, ESPN published Sudoku puzzles substituting the positions on a baseball field for the numbers 1 through 9; and Viz magazine published a Doctor Who version of the game, using images of the television series' first nine leading actors in place of the numerals. Dell Magazines, the puzzle's originator, has been using numerals for Number Place in its magazines since they first published it in 1979.