The Collaborative International Dictionary of English v.0.48:

left wing \left wing\ n.
   Those trying to reform the established order, particularly in
   political matters, to enhance the condition of the common
   people; -- sometimes applied to those advocating overthrow of
   an existing government by radical means.

   Syn: left, left wing, leftists, political left.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

left wing
    n 1: those who support varying degrees of social or political or
         economic change designed to promote the public welfare
         [syn: left, left wing]