Bellwether

A market bellwether is a security whose changing price is considered a signal that the market is changing direction.

It gets its name from the wether, or castrated ram, that walks at the head of a shepherd's flock. The distinctive tone of the bell around the wether's neck signals the flock's position.

There's not an official list of these trend setters, or market barometers, and they do change as the overall markets and the fortunes of individual companies change.