The Collaborative International Dictionary of English v.0.48:

Unionism \Un"ion*ism\, n.
   1. The sentiment of attachment to a federal union, especially
      to the federal union of the United States.
      [1913 Webster]

   2. The principles, or the system, of combination among
      workmen engaged in the same occupation or trade.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unionism
    n 1: the system or principles and theory of labor unions [syn:
         unionism, trade unionism]