The Collaborative International Dictionary of English v.0.48:

Federalist \Fed"er*al*ist\, n. [Cf. F. f['e]d['e]raliste.]
   An advocate of confederation; specifically (Amer. Hist.), a
   friend of the Constitution of the United States at its
   formation and adoption; a member of the political party which
   favored the administration of president Washington.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Federalist
    n 1: a member of a former political party in the United States
         that favored a strong centralized federal government
    2: an advocate of federalism