The Collaborative International Dictionary of English v.0.48:

Solidarity \Sol`i*dar"i*ty\, n. [F. solidarit['e], fr. solide.
   See Solid.]
   An entire union or consolidation of interests and
   responsibilities; fellowship; community.
   [1913 Webster]

         Solidarity [a word which we owe to the French
         Communists], signifies a fellowship in gain and loss,
         in honor and dishonor, in victory and defeat, a being,
         so to speak, all in the same boat.       --Trench.
   [1913 Webster]

         The solidarity . . . of Breton and Welsh poetry. --M.
                                                  Arnold.
   [1913 Webster]

WordNet (r) 3.0 (2006):

solidarity
    n 1: a union of interests or purposes or sympathies among
         members of a group

Moby Thesaurus II by Grady Ward, 1.0:

147 Moby Thesaurus words for "solidarity":
   accord, accordance, affinity, agape, agreement, amity,
   bipartisanship, bonds of harmony, brotherly love, camaraderie,
   caritas, cement of friendship, charity, coaction, coadjuvancy,
   coadministration, coagency, cochairmanship, codirectorship,
   cohesion, cohesiveness, collaboration, collaborativeness,
   collectivism, collusion, combination, commensalism, common effort,
   common enterprise, communalism, communion, communism,
   communitarianism, community, community of interest,
   community of interests, compatibility, completeness, complicity,
   comprehensiveness, comradeship, concert, concord, concordance,
   concurrence, congeniality, cooperation, cooperativeness,
   correspondence, duet, duumvirate, ecumenicalism, ecumenicism,
   ecumenism, empathy, entireness, entirety, esprit, esprit de corps,
   exhaustiveness, feeling of identity, fellow feeling, fellowship,
   firmness, fixity, frictionlessness, fusion, good vibes,
   good vibrations, happy family, harmony, identity, inclusiveness,
   individuality, indivisibility, intactness, integrality,
   integration, integrity, inviolability, irreducibility,
   joining of forces, joint effort, joint operation, kinship,
   like-mindedness, love, mass action, morale, mutual assistance,
   mutualism, mutuality, octet, omnipresence, oneness, organic unity,
   particularity, peace, pervasiveness, pooling, pooling of resources,
   pulling together, purity, quartet, quintet, rapport, rapprochement,
   reciprocity, selfsameness, septet, sextet, sharing, simplicity,
   single-mindedness, singleness, singleness of purpose, singularity,
   sodality, solidification, solidity, symbiosis, sympathy, symphony,
   synergism, synergy, team spirit, teamwork, thoroughness,
   togetherness, totality, trio, triumvirate, troika, ubiquity,
   unanimity, understanding, undividedness, unification, uniformity,
   union, uniqueness, unison, united action, unity, universality,
   univocity, wholeness