a number puzzle in which the numbers 1 through 9 must be placed into a grid of cells so that each row or column contains only one of each number
"She spent an hour solving a difficult sudoku by carefully placing numbers until every row and column contained exactly one of each digit from one to nine."
A type of pencil puzzle played on a square grid, requiring each of the columns, each of the rows, and each of the subregions (called "boxes" or "regions") to contain the lowest whole numbers up to the dimension of the puzzle, usually 1 to 9, once each. The subregions are rectangular or square except in variants.
"After carefully filling in the empty cells on the nine-by-nine grid, she realized her mistake when two nines appeared in the same box."
In plain English: Sudoku is a number puzzle where you fill a grid with digits so that each row, column, and small box contains all numbers from 1 to 9 without repeating any of them.
"She spent her entire vacation solving sudoku puzzles in coffee shop magazines."
Usage: The term refers specifically to the logic puzzle itself; avoid using it as a verb. It is often confused with similar grid-based games like crosswords or nonograms, but its rules are distinct in requiring numbers 1 through 9 without repetition.
The word sudoku comes directly from the Japanese phrase suudoku, which is a contraction of sūzu ("number") and dokoro ("place"). It entered English as the name for the popular logic puzzle that originated in Japan.