red flag

1. N-COUNT A red flag is a flag that is red in colour and is used as a symbol to represent communism and socialism or to indicate danger or as a sign that you should stop. (代表共产主义或社会主义的)红旗; (示意停止的危险信号)红旗

Then the rain came and the red flag went up to signal a halt. 当时，雨下了起来，红旗被升起，发出停止的信号。

2. N-COUNT If you refer to something as a red flag, you mean that it acts as a danger signal. 危险信号

The abnormal bleeding is your body's own red flag of danger. 这反常的流血是你身体本身的危险信号。