Bellwether: An event or indicator that shows the possible presence of a trend in overall market or sector direction. The performance of certain companies/stocks and bonds are considered by analysts to indicate the condition of the economy and financial markets because their performance is well-correlated with a trend. Bellwether companies are usually the market leaders in their respective sectors. The term is a combination of “bell” and “wether.” Shepherds would often hang bells around the necks of the wether (male sheep) that led the flock, in order to find them.
Source: Investor Glossary, Investopedia