Source : Webster's Revised Unabridged Dictionary (1913)

Seconder \Sec"ond*er\, n.
   One who seconds or supports what another attempts, affirms,
   moves, or proposes; as, the seconder of an enterprise or of a
   motion.

Source : WordNet®

seconder
     n : someone who endorses a motion or petition as a necessary
         preliminary to a discussion or vote