Bellwether is a noun that refers to an indicator or predictor of something.

Coming from Middle English, our word of the day began as a term for a sheep that lead the flock. More recently it’s used to refer to something that predicts a trend or new direction.

Example: I hope my son’s funky mustache isn’t a bellwether of trends in facial hair. As someone who survived the 70s, I wouldn’t like to see any of those mustaches came back into fashion.