The Collaborative International Dictionary of English v.0.48:

Communion \Com*mun"ion\, n. [L. communio: cf. F. communion. See
   Common.]
   1. The act of sharing; community; participation. "This
      communion of goods." --Blackstone.
      [1913 Webster]

   2. Intercourse between two or more persons; esp., intimate
      association and intercourse implying sympathy and
      confidence; interchange of thoughts, purposes, etc.;
      agreement; fellowship; as, the communion of saints.
      [1913 Webster]

            We are naturally induced to seek communion and
            fellowship with others.               --Hooker.
      [1913 Webster]

            What communion hath light with darkness? --2 Cor.
                                                  vi. 14.
      [1913 Webster]

            Bare communion with a good church can never alone
            make a good man.                      --South.
      [1913 Webster]

   3. A body of Christians having one common faith and
      discipline; as, the Presbyterian communion.
      [1913 Webster]

   4. The sacrament of the eucharist; the celebration of the
      Lord's supper; the act of partaking of the sacrament; as,
      to go to communion; to partake of the communion; called
      also Holy Communion.
      [1913 Webster +PJC]

   Close communion. See under Close, a.

   Communion elements, the bread and wine used in the
      celebration of the Lord's supper.

   Communion service, the celebration of the Lord's supper, or
      the office or service therefor.

   Communion table, the table upon which the elements are
      placed at the celebration of the Lord's supper.

   Communion in both kinds, participation in both the bread
      and wine by all communicants.

   Communion in one kind, participation in but one element, as
      in the Roman Catholic Church, where the laity partake of
      the bread only.

   Syn: Share; participation; fellowship; converse; intercourse;
        unity; concord; agreement.
        [1913 Webster]

WordNet (r) 3.0 (2006):

Communion
    n 1: the act of participating in the celebration of the
         Eucharist; "the governor took Communion with the rest of
         the congregation" [syn: Communion, Holy Communion,
         sacramental manduction, manduction]
    2: sharing thoughts and feelings [syn: communion, sharing]
    3: (Christianity) a group of Christians with a common religious
       faith who practice the same rites

Moby Thesaurus II by Grady Ward, 1.0:

209 Moby Thesaurus words for "communion":
   Angelus, Ave, Ave Maria, Bolshevik, Bolshie, Communist, ESP,
   Hail Mary, Kyrie Eleison, Leninist, Marxist, Paternoster, Red,
   Stalinist, Trotskyist, accord, accordance, adherents, affiliation,
   affinity, agape, agreement, aid prayer, amity, answer, appeal,
   balance, beadroll, beads, beseechment, bidding prayer, body,
   bonds of harmony, branch, breviary, brotherly love, caritas,
   cement of friendship, chaplet, charity, church, class, coequality,
   collect, collective farm, collectivism, collectivity, collegiality,
   colloquial discourse, colloquy, commerce, commie, common ownership,
   communal effort, communication, communism, community,
   community of interests, compatibility, comrade, concord,
   concordance, confession, congeniality, congress, connection,
   contact, contemplation, conversation, converse, conversing,
   cooperation, cooperative society, corelation, correlation,
   correlativism, correlativity, correspondence, creed, cult, dealing,
   dealings, democracy, denomination, devotions, disciples, discourse,
   division, empathy, entreaty, equilibrium, equipollence,
   equivalence, esprit, esprit de corps, exchange, faction, faith,
   feeling of identity, fellow feeling, fellow traveler, fellowship,
   followers, frictionlessness, give-and-take, good vibes,
   good vibrations, grace, group, happy family, harmony, identity,
   impetration, imploration, information, interaction, intercession,
   interchange, intercommunication, intercommunion, intercourse,
   interlocution, interplay, inverse proportion, inverse ratio,
   inverse relationship, invocation, ism, kibbutz, kinship, kolkhoz,
   like-mindedness, linguistic intercourse, litany, love, meditation,
   message, mutuality, obsecration, obtestation, offshoot, oneness,
   order, organization, orison, party, peace, persuasion, petition,
   pink, pinko, prayer, prayer wheel, profit sharing, proportionality,
   public ownership, rapping, rapport, rapprochement, reciprocality,
   reciprocation, reciprocity, relativity, religious order, reply,
   response, rogation, rosary, schism, school, sect, sectarism,
   segment, sharecropping, sharing, silent prayer, social activity,
   social intercourse, social relations, socialism, society,
   solidarity, speaking, speech, speech circuit, speech situation,
   state ownership, suit, supplication, symmetry, sympathy, symphony,
   talking, team spirit, telepathy, thanks, thanksgiving, touch,
   town meeting, traffic, truck, two-way communication, understanding,
   union, unison, unity, variety, verbal intercourse, version



Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "Communion":
   Agnus Dei, Alleluia, Anamnesis, Blessing, Canon, Collect,
   Consecration, Credo, Dismissal, Epistle, Eucharist, Fraction,
   Gloria, Gospel, Gradual, Holy Communion, Host, Introit, Kyrie,
   Kyrie Eleison, Last Gospel, Last Supper, Lavabo, Offertory,
   Paternoster, Pax, Post-Communion, Preface, Sacrament Sunday,
   Sanctus, Secreta, Tersanctus, Tract, altar bread, bread,
   bread and wine, consecrated bread, consecrated elements,
   consubstantiation, elements, impanation, intinction, loaf,
   real presence, subpanation, the Holy Sacrament, the Sacrament,
   transubstantiation, wafer



Easton's 1897 Bible Dictionary:

Communion
   fellowship with God (Gen. 18:17-33; Ex. 33:9-11; Num. 12:7, 8),
   between Christ and his people (John 14:23), by the Spirit (2
   Cor. 13:14; Phil. 2:1), of believers with one another (Eph.
   4:1-6). The Lord's Supper is so called (1 Cor. 10:16, 17),
   because in it there is fellowship between Christ and his
   disciples, and of the disciples with one another.