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]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "left wing":
   Gospel side, cantorial side, counterclockwise, larboard, left,
   left hand, left of center, left-hand, left-hand side, left-winger,
   left-wingish, leftism, leftist, levorotatory, liberal, liberalism,
   liberalist, liberalistic, near, near side, nigh, on the left, port,
   port tack, portside, progressive, progressivism, progressivistic,
   radical, sinister, sinistral, sinistrocerebral, sinistrocular,
   sinistrogyrate, sinistrorse, verso, wrong side