"A bellwether; one that leads or indicates trends.
In the stock market, a bellwether is the stock of a company that is regarded as a leader in its given industry. The performance of the stock is said to reflect the performance of the industry in general. These stocks are used as bellwethers for the rest of the market.
In sociology, the term is applied in the active sense to a person or group of people who tend to create, influence, or set trends."