bell·weth·er
   /ˈbɛlˌwɛðər/ Show Spelled[bel-weth-er] Show IPA

noun
1.
a wether or other male sheep that leads the flock, usually bearing a bell.
2.
a person or thing that assumes the leadership or forefront, as of a profession or industry: Paris is a bellwether of the fashion industry.
3.
a person or thing that shows the existence or direction of a trend; index.
4.
a person who leads a mob, mutiny, conspiracy, or the like; ringleader.