The Collaborative International Dictionary of English v.0.48:

Federation \Fed`er*a"tion\, n. [Cf. F. f['e]d['e]ration.]
   1. The act of uniting in a league; confederation.
      [1913 Webster]

   2. A league; a confederacy; a federal or confederated
      government. --Burke.
      [1913 Webster]

WordNet (r) 3.0 (2006):

federation
    n 1: an organization formed by merging several groups or parties
    2: a union of political organizations [syn: confederation,
       confederacy, federation]
    3: the act of constituting a political unity out of a number of
       separate states or colonies or provinces so that each member
       retains the management of its internal affairs

Moby Thesaurus II by Grady Ward, 1.0:

160 Moby Thesaurus words for "federation":
   Anschluss, Bund, Rochdale cooperative, absolute monarchy, addition,
   affiliation, agglomeration, aggregation, agreement, alignment,
   alliance, amalgamation, anschluss, aristocracy, assemblage,
   assimilation, association, autarchy, autocracy, autonomy, axis,
   band, blend, blending, bloc, body, cabal, cahoots, cartel,
   centralization, coadunation, coalescence, coalition,
   coalition government, colleagueship, college, collegialism,
   collegiality, colonialism, combination, combine, combo,
   common market, commonwealth, composition, comradeship, confederacy,
   confederation, confraternity, congeries, conglomeration,
   conjugation, conjunction, consolidation, conspiracy,
   constitutional government, constitutional monarchy,
   consumer cooperative, cooperative, cooperative society,
   copartnership, copartnery, corps, council, credit union,
   customs union, democracy, dictatorship, dominion rule, duarchy,
   duumvirate, dyarchy, economic community, ecumenism, embodiment,
   encompassment, enosis, federal government, federalization,
   fellowship, feudal system, fraternalism, fraternity,
   fraternization, free trade area, freemasonry, fusion, gang,
   garrison state, gerontocracy, group, grouping, heteronomy,
   hierarchy, hierocracy, home rule, hookup, inclusion, incorporation,
   integration, junction, junta, league, limited monarchy, machine,
   marriage, martial law, meld, melding, merger, meritocracy,
   militarism, military government, mob, mob rule, mobocracy,
   monarchy, neocolonialism, ochlocracy, oligarchy, package,
   package deal, pantisocracy, partnership, patriarchate, patriarchy,
   police state, political machine, pure democracy, regency,
   representative democracy, representative government, republic,
   ring, self-determination, self-government, social democracy,
   society, sodality, solidification, sorority, stratocracy,
   syncretism, syndication, syneresis, synthesis, technocracy,
   thearchy, theocracy, tie-in, tie-up, totalitarian government,
   totalitarian regime, triarchy, triumvirate, tyranny, unification,
   union, wedding, welfare state



The Free On-line Dictionary of Computing (30 December 2018):

federation

    The establishment of some or all of business
   agreements, cryptographic trust and user identifiers or
   attributes across security and policy domains to enable more
   seamless business interaction.

   As web services promise to enable integration between
   business partners through loose coupling at the application
   and messaging layer, federation does so at the identity
   management layer, insulating each domain from the details of
   the others' authentication and authorization.  Key to this
   loose coupling at the identity management layer are
   standardized mechanisms and formats for the communication of
   identity information between the domains.  SAML is one such
   standard.

   (2011-05-12)