The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            A primacy of order, such an one as the ringleader
            hath in a dance.                      --Barrow.
      [1913 Webster]

   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.
      [1913 Webster]

            The ringleaders were apprehended, tried, fined, and
            imprisoned.                           --Macaulay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ringleader
    n 1: a person who leads (especially in illicit activities)