The Collaborative International Dictionary of English v.0.48:

Loyalist \Loy"al*ist\, n.
   A person who adheres to his sovereign or to the lawful
   authority; especially, one who maintains his allegiance to
   his prince or government, and defends his cause in times of
   revolt or revolution.
   [1913 Webster]

WordNet (r) 3.0 (2006):

loyalist
    n 1: a person who is loyal to their allegiance (especially in
         times of revolt) [syn: loyalist, stalwart]