Definition:Perfect Field/Definition 2

Definition
Let $F$ be a field.

$F$ is a perfect field one of the following holds:
 * $\Char F = 0$
 * $\Char F = p$ with $p$ prime and $\Frob$ is an automorphism of $F$

where:
 * $\Char F$ denotes the characteristic of $F$
 * $\Frob$ denotes the Frobenius endomorphism on $F$

Also see

 * Equivalence of Definitions of Perfect Field
 * Frobenius Endomorphism on Field is Injective
 * Definition:Separably Closed Field, a field with no separable extensions

Examples

 * Galois Field is Perfect
 * Algebraically Closed Field is Perfect