Source : Webster's Revised Unabridged Dictionary (1913)

Bellwether \Bell"weth`er\, n.
   1. A wether, or sheep, which leads the flock, with a bell on
      his neck.

   2. Hence: A leader. [Contemptuous] --Swift.

Source : WordNet®

bellwether
     n 1: someone who assumes leadership of a movement or activity
     2: sheep that leads the herd often wearing a bell