A share (or broadly a security) that is widely perceived by the market to be a representative of a specific industry or the entire market. Such a share is considered to be leading the market, and in it is a common clue as to the direction of future price trends. For example, if the price of a bellwether share is moving up, then the whole market would be considered to setting on a bullish trajectory.

Examples of bellwether shares include those of General Motors and IBM (previously), and Microsoft and Apple (in the present).

A bellwether share is often seen to be a thermostat of the state of the stock market, and therefore  technical analysts tend to keep it under surveillance in their attempts to predict market movements.