Definition of sudoku noun from the Oxford Advanced American Dictionary

 

sudoku

 noun
noun
NAmE//suˈdoʊku//
 
[countable, uncountable]
 
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a number puzzle with nine squares, each containing nine smaller squares, in which you have to put the numbers one to nine so that a number appears only once in each of the nine squares and in each row of nine across and down the puzzle He passes the time doing sudokus. There are sudokus at four levels of difficulty.
See the Oxford Advanced Learner's Dictionary entry: sudoku