FACTORIAL

in mathematics, the product of all positive integers less than or equal to a given number and denoted by that number and an exclamation point. Thus, factorial seven is written 7!, meaning 1 2 3 4 5 6 7. Factorial zero is arbitrarily defined as equal to 1, despite the fact that 1 also is the value of factorial one. Factorials are commonly encountered in the evaluation of permutations and combinations and in the coefficients of terms of binomial expansions. See also binomial theorem; permutations and combinations.