WordNet (r) 3.0 (2006):

ecumenism
    n 1: a movement promoting union between religions (especially
         between Christian churches) [syn: ecumenism,
         oecumenism]
    2: (Christianity) the doctrine of the ecumenical movement that
       promotes cooperation and better understanding among different
       religious denominations: aimed at universal Christian unity
       [syn: ecumenism, ecumenicism, ecumenicalism]

Moby Thesaurus II by Grady Ward, 1.0:

133 Moby Thesaurus words for "ecumenism":
   Anschluss, addition, affiliation, agglomeration, aggregation,
   agreement, alliance, amalgamation, assimilation, association,
   bipartisanship, blend, blending, breadth, broad gauge,
   broad-mindedness, broadness, cabal, cartel, centralization,
   coaction, coadjuvancy, coadministration, coagency, coalescence,
   coalition, cochairmanship, codirectorship, collaboration,
   collaborativeness, collectivism, collusion, combination, combine,
   combo, commensalism, common effort, common enterprise, communalism,
   communism, communitarianism, community, complicity, composition,
   concert, concord, concordance, concurrence, confederacy,
   confederation, congeries, conglomeration, conjugation, conjunction,
   consolidation, conspiracy, cooperation, cooperativeness,
   cosmopolitanism, duet, duumvirate, ecumenicalism, ecumenicism,
   ecumenicity, embodiment, encompassment, enosis, esprit,
   esprit de corps, federalization, federation, fellow feeling,
   fellowship, fusion, harmony, hookup, inclusion, incorporation,
   integration, joining of forces, joint effort, joint operation,
   junction, junta, large-mindedness, latitude, league, marriage,
   mass action, meld, melding, merger, morale, mutual assistance,
   mutualism, mutuality, noninsularity, octet, package, package deal,
   pooling, pooling of resources, pulling together, quartet, quintet,
   reciprocity, septet, sextet, solidarity, solidification,
   spacious mind, symbiosis, syncretism, syndication, syneresis,
   synergism, synergy, synthesis, team spirit, teamwork, tie-up, trio,
   triumvirate, troika, unbigotedness, unhideboundness, unification,
   union, united action, unparochialism, unprovincialism, wedding,
   wide-mindedness