Abstract Algebra
《Abstract Algebra》 · 第 650 页 · Section 14.9「Transcendental Extensions, Inseparable Extensions, Infinite Galois Groups」，Proposition 后的定义段
Let $E/F$ be an algebraic extension. Then there is a unique field $E_{\mathrm{sep}}$ with $F\subseteq E_{\mathrm{sep}}\subseteq E$ such that $E_{\mathrm{sep}}$ is separable over $F$ and $E$ is purely inseparable over $E_{\mathrm{sep}}$. The field $E_{\mathrm{sep}}$ is the set of elements of $E$ which are separable over $F$. The degree of $E_{\mathrm{sep}}/F$ is called the separable degree of $E/F$ and the degree of $E/E_{\mathrm{sep}}$ is called the inseparable degree of $E/F$ (often denoted as $[E:F]_s$ and $[E:F]_i$ respectively).