An
approximation to the
factorial function.
N! ~ NN e-N
or, in logarithmic form:
ln N! = N ln N - N
For large N, this is a good approximation to the factorial.
With some adjustments it is possible to make it better, but since it is mostly used for back-of-the-envelope types of calculations, this is usually good enough.
This is based on a truncation of Stirling's Series, an infinite series which converges to exactly the value of the factorial.