The number 81 is a whole number that follows 80 and precedes 82. It is an integer and can be classified as a composite number because it has factors other than 1 and itself. Specifically, 81 can be divided evenly by 1, 3, 9, 27, and 81. In mathematics, 81 is also known as a perfect square because it is the square of 9 (9 x 9 = 81). This property makes it significant in various mathematical concepts, including geometry and algebra. Additionally, 81 is often used in games and puzzles, such as Sudoku, where it represents the total number of cells in a standard grid.