Hattie Cobb's List: computer

A bellwether is any entity in a given arena that serves to create or influence trends or to presage future happenings.

The term is derived from the Middle English bellewether and refers to the practice of placing a bell around the neck of a castrated ram (a wether) leading his flock of sheep.^[1][2] The movements of the flock could be noted by hearing the bell before the flock was in sight.