What is a Bellwether?
A bellwether is a security or indicator that signals the market's 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.
A bellwether stock is a stock that is used to gauge the performance of the market in general. For many years General Motors was an example of a bellwether stock, hence the saying, "What's good for GM is good for America." Similarly, a rapid decrease in available steel may indicate economic recovery, since steel is used in manufacturing and building.