Source : Webster's Revised Unabridged Dictionary (1913)

Rioter \Ri"ot*er\, n.
   1. One who riots; a reveler; a roisterer. --Chaucer.

   2. (Law) One who engages in a riot. See {Riot}, n., 3.

Source : WordNet®

rioter
     n : troublemaker who participates in a violent disturbance of
         the peace; someone who rises up against the constituted
         authority