A railway signal has various states, one of which is the most restrictive state. This is often the red aspect (or a horizontal semaphore arm) which means "stop" or "danger".

But because not all signals have a stop or danger state, the best term is "most restrictive" e.g. some signals have only caution or proceed states.

Signals are designed that if they fail, they will fail to their most restrictive state (fail safe).