bellwether

The leading sheep of a flock, having a bell hung round its neck.

Several old ladies forthwith proclaimed their intention of following him; but, as one or two of them were deaf, and another had been threatened with an attack of that mild, but obstinate complaint, dementia senilis, many thought it was not so much the force of his arguments as a kind of tendency to jump as the bellwether jumps, well known in flocks not included in the Christian fold.

Anything that indicates future trends.

Mortgage delinquencies often act as a bellwether for a forthcoming recession.

At Hampton Court the little flocks of visitors are not provided with an official bellwether, but are left to browse at discretion upon the local antiquities.

A stock or bond that is widely believed to be an indicator of the overall market's condition.

One of my favorite sentiment indicators is bellwether stocks — key stocks that are leaders in their fields. When investors buy or sell these shares, it's usually a good sign of confidence or the lack of it in that sector of the market and sometimes even the economy.