ramification

《Number Fields》 · 第 50 页 · Chapter 3「Prime Decomposition in Number Rings」，Definition

Let $K,L,R,$ and $S$ be as usual; a prime $P$ of $R$ is ramified in $S$ (or in $L$) iff $e(Q\mid P)>1$ for some prime $Q$ of $S$ lying over $P$. (In other words, $PS$ is not squarefree.)