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先兆；征兆；风向标
If you describe something as a bellwether, you mean that it is an indication of the way a situation is changing.
例句
For decades the company was the bellwether of the British economy...
几十年来，这家公司一直是英国经济的晴雨表。
IBM is considered the bellwether stock on Wall Street.
国际商用机器公司的股票行情被视为华尔街股市的风向标。