A factorial, denoted as $n!$, is the same thing as $n \times (n-1) \times (n-2) \times \cdots \times 2 \times 1$. In other words, it is the product of all positive integers between 1 and $n$, inclusive.
There are other variations of this, such as the double factorial ($n!!$), which basically means the product of all even integers between $n$ and 1, inclusive.
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