The Stacks project

Definition 9.14.7. Let $E/F$ be an algebraic field extension. Let $E_{sep}$ be the subextension found in Lemma 9.14.6.

1. The integer $[E_{sep} : F]$ is called the separable degree of the extension. Notation $[E : F]_ s$.

2. The integer $[E : E_{sep}]$ is called the inseparable degree, or the degree of inseparability of the extension. Notation $[E : F]_ i$.