Puzzle type that was the Oxford Dictionaries' 2005 U.K. "word of the year"

SUDOKU

This number-placement game from Japan typically uses a 9x9 grid where each row, column, and 3x3 box must contain digits 1 through 9 exactly once.

This clue refers to the number-based puzzle that surged in popularity in the mid-2000s. Sudoku was named the Oxford Dictionaries' 2005 U.K. "word of the year" because it became a cultural phenomenon, appearing in newspapers and books worldwide. The answer is straightforward—no wordplay here—just a direct reference to the puzzle’s recognition during that time.