The Collaborative International Dictionary of English v.0.48:

leftist \leftist\ adj.
   same as left-wing.

   Syn: left-of-center, left-wing.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

leftist \leftist\ n.
   a person who belongs to the political left.

   Syn: socialist, collectivist, left-winger.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

leftist
    adj 1: believing in or supporting tenets of the political left
           [syn: leftist, left-of-center, left-wing]
    n 1: a person who belongs to the political left [syn:
         collectivist, leftist, left-winger]