The Collaborative International Dictionary of English v.0.48:

Emancipationist \E*man`ci*pa"tion*ist\, n.
   An advocate of emancipation, esp. the emancipation of slaves.
   [1913 Webster]

WordNet (r) 3.0 (2006):

emancipationist
    n 1: a reformer who favors abolishing slavery [syn:
         abolitionist, emancipationist]