A bellwether is a leader or an indicator of trends.[1]
In politics, the term often applies in a metaphorical sense to characterize a geographic region where political tendencies match in microcosm those of a wider area, such that the result of an election in the former region might predict the eventual result in the latter. In economics, a 'bellwether' is a leading indicator of an economic trend.[1][2]
Sociologists apply the term in the active sense to a person or group of people who tend to create, influence, or set trends.