Definition 1

— Let E be an algebraic extension of K; then E is said to be separable (over K) if every subextension F of E of finite degree over K is an etale algebra over K ($V$, p. 28, Def. 1).

Let E be an extension of finite degree of K. Since every subalgebra of an etale algebra is etale ($V$, p. 30, Prop. 3), it comes to the same to suppose that E is a separable extension of $K$, or that E is an etale algebra over $K$.