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The factorial of a whole number \displaystyle{ n }, written as \displaystyle{ n! } or \displaystyle{ n }, is found by multiplying \displaystyle{ n } by all the whole numbers less than it. For example, the factorial of 4 is 24, because \displaystyle{ 4 \times 3 \times 2 \times 1 = 24 }. Hence one can write \displaystyle{ 4! = 24 }. For some technical reasons, 0! is equal to 1.

Factorials can be used to find out how many possible ways there are to arrange \displaystyle{ n } objects.