Source : Webster's Revised Unabridged Dictionary (1913)

Ringleader \Ring"lead`er\, n.
   1. The leader of a circle of dancers; hence, the leader of a
      number of persons acting together; the leader of a herd of
      animals.

            A primacy of order, such an one as the ringleader
            hath in a dance.                      --Barrow.

   2. Opprobriously, a leader of a body of men engaged in the
      violation of law or in an illegal enterprise, as rioters,
      mutineers, or the like.

            The ringleaders were apprehended, tried, fined, and
            imprisoned.                           --Macaulay.

Source : WordNet®

ringleader
     n : a person who leads (especially in illicit activities)