The expression "1!" represents the factorial of the number 1. In mathematics, the factorial of a non-negative integer is the product of all positive integers up to that number. For example, the factorial of 3, written as 3!, is calculated as 3 × 2 × 1, which equals 6. For the number 1, the factorial is simply 1, since there are no other positive integers to multiply. Therefore, 1! = 1. Factorials are commonly used in various areas of mathematics, including combinatorics and probability theory, to calculate permutations and combinations.