An irreducible polynomial $f \in F[x]$ with coefficients in a field $F$ is separable if $f$ factors into distinct linear factors over a splitting field $K$ of $f$

A polynomial $g$ with coefficients in $F$ is separable if each irreducible factor of $g$ in $F[x]$ is a separable polynomial.

An algebraic field extension $K/F$ is separable if, for each $a \in K$ the minimal polynomial of $a$ over $F$ is separable. When $F$ has characteristic zero, every algebraic extension of $F$ is separable; examples of inseparable extensions include the quotient field $K(u)[t]/(t^p-u)$ over the field $K(u)$ of rational functions in one variable, where $K$ has characteristic $p > 0$

More generally, an arbitrary field extension $K/F$ is defined to be separable if every finitely generated intermediate field extension $L/F$ has a transcendence basis $S \subset L$ such that $L$ is a separable algebraic extension of $F(S)$