Found in the following dictionaries
OTCWL2014 (NASPA Word List 2014)
TWL06 (NASPA List 2006)
TWL06 (NASPA List 2006)
"Sudoku" Definition
noun
A puzzle in which a grid consisting of several regions is to be filled with numbers so that every row, column, and region contains only one instance of each number. The most common format is a grid of nine rows and columns that are divided into nine smaller regions of three rows and three columns into which the numbers 1 through 9 must be placed.
Source: American Heritage Dictionary
A puzzle in which several numbers are to be filled into a 9x9 grid of squares so that every row, every column, and every 3x3 box contains the numbers 1 through 9.
Source: Merriam-Webster