The term 'Popular Number Puzzle' is used to refer to the popular Japanese puzzle game Sudoku. Sudoku is a logic-based number placement game where players must fill in a 9x9 grid with numbers from 1 to 9, such that each row, column, and 3x3 sub-grid contains all of the digits from 1 to 9. The objective of the game is to fill in the grid with the correct numbers without repeating any. Sudoku has become a popular pastime around the world and is enjoyed by people of all ages.