Algebraic Number Theory
《Algebraic Number Theory》 · 第 175 页 · Chapter II，Section 9「Galois Theory of Valuations」，Definition 9.13
The fixed field of $R_w$, $V_w=V_w(L/K)=\{x\in L\mid \sigma x=x\text{ for all }\sigma\in R_w\}$, is called the ramification field of $w$ over $K$.