Source : Webster's Revised Unabridged Dictionary (1913)

Federate \Fed"er*ate\, a. [L. foederatus, p. p. of foederare to
   establish by treaty or league, fr. foedus. See {Federal}.]
   United by compact, as sovereignties, states, or nations;
   joined in confederacy; leagued; confederate; as, federate
   nations.

Source : WordNet®

federate
     v 1: enter into a league for a common purpose; "The republics
          federated to become the Soviet Union" [syn: {federalize},
           {federalise}]
     2: unite on a federal basis or band together as a league; "The
        country was federated after the civil war" [syn: {federalize},
         {federalise}]

federate
     adj : united under a central government [syn: {federated}]