Which is not a square number 9 16 36 48 81 100?

The number 48 is not a square number because it cannot be expressed as the product of an integer multiplied by itself. Square numbers are numbers that can be represented as n^2, where n is an integer. In this case, 48 cannot be expressed as the square of an integer, unlike 9, 16, 36, 81, and 100, which are all square numbers.