The Collaborative International Dictionary of English v.0.48:

Unionist \Un"ion*ist\, n.
   1. One who advocates or promotes union; especially a loyal
      supporter of a federal union, as that of the United
      States.
      [1913 Webster]

   2. A member or supporter of a trades union.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unionist
    n 1: a worker who belongs to a trade union [syn: trade
         unionist, unionist, union member]