Fermat's Little Theorem

Theorem
Let $p$ be a prime number.

Let $n \in \Z_{>0}$ be a positive integer such that $p$ is not a divisor of $n$.

Then:
 * $n^{p - 1} \equiv 1 \pmod p$

Also known as
Some sources call this Fermat's Theorem, but it needs to be appreciated that this may cause confusion with Fermat's Last Theorem.

Also defined as
Some sources refer to the corollary $n^p \equiv n \pmod p$ as Fermat's little theorem and from it derive this result.