The Collaborative International Dictionary of English v.0.48:

Affiliation \Af*fil`i*a"tion\, n. [F. affiliation, LL.
   affiliatio.]
   1. Adoption; association or reception as a member in or of
      the same family or society.
      [1913 Webster]

   2. (Law) The establishment or ascertaining of parentage; the
      assignment of a child, as a bastard, to its father;
      filiation.
      [1913 Webster]

   3. Connection in the way of descent. --H. Spencer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

affiliation
    n 1: a social or business relationship; "a valuable financial
         affiliation"; "he was sorry he had to sever his ties with
         other members of the team"; "many close associations with
         England" [syn: affiliation, association, tie, tie-
         up]
    2: the act of becoming formally connected or joined; "welcomed
       the affiliation of the research center with the university"

Moby Thesaurus II by Grady Ward, 1.0:

216 Moby Thesaurus words for "affiliation":
   Americanization, Anschluss, acceptance, accord, acculturation,
   addition, adjunct, admission, adoption, affairs, affinity,
   agglomeration, aggregation, agnation, agreement, alignment,
   alliance, amalgamation, ancestry, apparentation, approximation,
   assemblage, assimilation, association, birth, blend, blending,
   blood, blood relationship, bloodline, body, bond, branch, breed,
   brotherhood, brothership, cabal, cahoots, cartel, centralization,
   church, citizenship by naturalization, citizenship papers,
   closeness, coadunation, coalescence, coalition, cognation,
   colleagueship, collegialism, collegiality, combination, combine,
   combo, common ancestry, common descent, communion, community,
   companionship, company, composition, comradeship, confederacy,
   confederation, confraternity, congeries, conglomeration,
   conjugation, conjunction, connectedness, connection, consanguinity,
   consociation, consolidation, conspiracy, contiguity, contrariety,
   cooperation, copartnership, copartnery, cousinhood, cousinship,
   culture shock, dealings, deduction, denomination, derivation,
   descent, direct line, disjunction, distaff side, division,
   ecumenism, embodiment, embracement, enation, encompassment, enosis,
   espousal, extraction, faction, family, fatherhood, federalization,
   federation, fellowship, female line, filiation, fraternalism,
   fraternity, fraternization, freemasonry, fusion, group, homology,
   hookup, house, inclusion, incorporation, integration, intercourse,
   intimacy, junction, junta, kindred, kinship, league, liaison, line,
   line of descent, lineage, link, linkage, linking, male line,
   marriage, maternity, matrilineage, matriliny, matrisib, matrocliny,
   meld, melding, membership, merger, motherhood, mutual attraction,
   nationalization, naturalization, naturalized citizenship, nearness,
   offshoot, order, organization, package, package deal, papers,
   partaking, participation, partnership, party, paternity,
   patrilineage, patriliny, patrisib, patrocliny, persuasion, phylum,
   propinquity, proximity, race, rapport, relatedness, relation,
   relations, relationship, religious order, schism, school, sect,
   sectarism, seed, segment, sept, sharing, sibship, side, similarity,
   sisterhood, sistership, society, sodality, solidification,
   sorority, spear side, spindle side, stem, stirps, stock, strain,
   succession, sword side, sympathy, syncretism, syndication,
   syneresis, synthesis, tie, tie-in, tie-up, ties of blood,
   unification, union, variety, version, wedding