Bellwether literally means a sheep that leads the herd, often bearing a bell; hence a share which assumes a position of market leadership. If it moves up, the market is buoyant, if it declines, the market falls. It is not, as some people mistakenly suppose, a share that is supposed to hold firm in stormy conditions.
The stock of a company recognized as a leader in its industry. For example, IBM is considered a bellwether stock in the computer field. Often, the fortunes of an industry are reflected in the behavior of its bellwether stocks.