The rare noun wether means a castrated male sheep. The word bellwether (from the Middle English bellewether) originally referred to a wether that wore a bell around his neck and led the herd. Today, while herders may still use the term in that sense, the word is more often used metaphorically, meaning one that serves as a leader or as a leading indicator of future trends. Bellwether may be a noun or an adjective.
The word is occasionally misspelled bellweather, perhaps partially because bellwethers sometimes predict the weather.
In each of these examples, the bellwether or the thing described by the adjective bellwether is an indicator of emerging or future trends:
Scranton is a bellwether city in a key swing state, and Obama has a lot of work to do there. [CBS News]
John Lewis, the department store chain that is a bellwether for retail demand in Britain, reported another disappointing set of sales, underlining doubts about future household spending. [Financial Times]
The 2008 election broke Missouri’s bellwether streak, as no candidate headed to the White House without carrying the state since 1956. [Politico]
Because the Golden Globes are held in January, the voters of the Hollywood Foreign Press Assn. get first crack at assessing new shows from the fall season. That often makes the results a bellwether. [Los Angeles Times]
Technology stocks fell after Hewlett-Packard, often considered a bellwether for the group, gave a disappointing revenue forecast for the current fiscal year. [Sydney Morning Herald]
As these examples demonstrate, bellwether is often used in financial and political contexts, but the word works elsewhere.