The Collaborative International Dictionary of English v.0.48:

left-wing \left-wing\ a.
   1. Of or pertaining to the political left or its ideology.
      [PJC]

   2. Believing in or supporting tenets of the political left.

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

WordNet (r) 3.0 (2006):

left-wing
    adj 1: believing in or supporting tenets of the political left
           [syn: leftist, left-of-center, left-wing]