Algebraic Number Theory
《Algebraic Number Theory》 · 第 168 页 · Chapter II，Section 9「Galois Theory of Valuations」，Definition 9.3
The ramification group is $R_w=R_w(L/K)=\{\sigma\in G_w\mid \sigma x/x\equiv1\pmod{\mathfrak P}\text{ for all }x\in L^*\}$.