A factorial, denoted by the symbol "n!", is the product of all positive integers from 1 to n. For example, 5! equals 5 × 4 × 3 × 2 × 1, which is 120. Factorials are commonly used in mathematics, particularly in permutations, combinations, and other areas of combinatorial analysis. By definition, the factorial of 0 is 1 (0! = 1).