Source : Webster's Revised Unabridged Dictionary (1913)

Jacobin \Jac"o*bin\, a.
   Same as {Jacobinic}.

Jacobin \Jac"o*bin\, n. [F. See 2d {Jack}, {Jacobite}.]
   1. (Eccl. Hist.) A Dominican friar; -- so named because,
      before the French Revolution, that order had a convent in
      the Rue St. Jacques, Paris.

   2. One of a society of violent agitators in France, during
      the revolution of 1789, who held secret meetings in the
      Jacobin convent in the Rue St. Jacques, Paris, and
      concerted measures to control the proceedings of the
      National Assembly. Hence: A plotter against an existing
      government; a turbulent demagogue.

   3. (Zo["o]l.) A fancy pigeon, in which the feathers of the
      neck form a hood, -- whence the name. The wings and tail
      are long, and the beak moderately short.

Black friar \Black" fri`ar\ (Eccl.)
   A friar of the Dominican order; -- called also {predicant}
   and {preaching friar}; in France, {Jacobin}. Also, sometimes,
   a Benedictine.

Source : WordNet®

Jacobin
     n : a member of the radical movement that instituted the Reign
         of Terror during the French Revolution