The Collaborative International Dictionary of English v.0.48:

Revolutionist \Rev`o*lu"tion*ist\, n.
   One engaged in effecting a change of government; a favorer of
   revolution. --Burke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

revolutionist
    n 1: a radical supporter of political or social revolution [syn:
         revolutionist, revolutionary, subversive,
         subverter]