The Collaborative International Dictionary of English v.0.48:

Demagogue \Dem"a*gogue\ (?; 115), n. [Gr. dhmagwgo`s a popular
   leader; commonly in a bad sense, a leader of the mob; dh^mos
   the people + 'agwgo`s leading, fr. 'a`gein to lead; akin to
   E. act: cf. F. d['e]magogue.]
   A leader of the rabble; one who attempts to control the
   multitude by specious or deceitful arts; an unprincipled and
   factious mob orator or political leader.
   [1913 Webster]

WordNet (r) 3.0 (2006):

demagogue
    n 1: a political leader who seeks support by appealing to
         popular passions and prejudices [syn: demagogue,
         demagog, rabble-rouser]