hear it again

: someone or something that leads the way or points out a trend

Example sentence:
Jenna was the school's fashion bellwether, and it was a sure thing that if she wore bell-bottoms on Monday, the whole school would be wearing them by Friday.

Etymology:
When "bellwether" was first used in the 1200s, it didn't conjure images of leaders like Genghis Khan conquering nations. Instead, it conjured images of sheep. That's because the word "wether" means "a male sheep." Flocks of sheep can be disorderly and hard to lead, so shepherds used to put a bell on the leader of the flock, usually a wether. "Bellwether" originally referred to this belled sheep. Because the bellwether was also the leader, the word "bellwether" soon came to be used for a leader, whether a sheep or not.