Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. A member or supporter of a trades union.

Source : WordNet®

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