The Collaborative International Dictionary of English v.0.48:

Symbiosis \Sym`bi*o"sis\, n. [NL., fr. Gr. symbi`wsis a living
   together, symbioy^n to live together; sy`n with + ? to live.]
   (Biol.)
   The living together in more or less imitative association or
   even close union of two dissimilar organisms. In a broad
   sense the term includes parasitism, or

   antagonistic symbiosis or

   antipathetic symbiosis, in which the association is
      disadvantageous or destructive to one of the organisms,
      but ordinarily it is used of cases where the association
      is advantageous, or often necessary, to one or both, and
      not harmful to either. When there is bodily union (in
      extreme cases so close that the two form practically a
      single body, as in the union of algae and fungi to form
      lichens, and in the inclusion of algae in radiolarians) it
      is called

   conjunctive symbiosis; if there is no actual union of the
      organisms (as in the association of ants with
      myrmecophytes),

   disjunctive symbiosis.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

symbiosis
    n 1: the relation between two different species of organisms
         that are interdependent; each gains benefits from the other
         [syn: symbiosis, mutualism]

Moby Thesaurus II by Grady Ward, 1.0:

129 Moby Thesaurus words for "symbiosis":
   accompaniment, accordance, agglomeration, agglutination,
   aggregation, agreement, alliance, articulation, association,
   bipartisanship, bond, bracketing, cahoots, clustering, co-working,
   coaction, coadjuvancy, coadministration, coagency, cochairmanship,
   codirectorship, coincidence, collaboration, collaborativeness,
   collectivism, collectivity, collusion, combination,
   combined effort, commensalism, commensality, common effort,
   common enterprise, communalism, communication, communism,
   communitarianism, community, complicity, concatenation, concert,
   concerted action, concomitance, concord, concordance, concourse,
   concurrence, confluence, congeries, conglomeration, conjugation,
   conjunction, connection, consilience, conspiracy, convergence,
   cooperation, cooperativeness, copulation, correspondence, coupling,
   duet, duumvirate, ecology, ecosystem, ecumenicalism, ecumenicism,
   ecumenism, esprit, esprit de corps, fellow feeling, fellowship,
   gathering, harmony, hookup, intercommunication, intercourse,
   interlinking, joinder, joining, joining of forces, joint effort,
   joint operation, jointure, junction, knotting, liaison, linkage,
   linking, marriage, mass action, meeting, merger, merging, morale,
   mutual assistance, mutualism, mutuality, octet, pairing,
   parasitism, pooling, pooling of resources, pulling together,
   quartet, quintet, reciprocity, saprophytism, septet, sextet,
   simultaneity, solidarity, splice, symbiotics, synchronism,
   synergism, synergy, team spirit, teamwork, tie, tie-in, tie-up,
   trio, triumvirate, troika, unification, union, united action,
   yoking