Permutations
A permutation of $n$ items is an ordered list of all of those items. The number of possible permutations of $n$ items is given by
$n$ factorial, given by
$n! = n \times (n-1) \times (n-2) \times ... \times 2 \times 1$
for $n$ a positive integer. We also take, by convention
$0! = 1.$