from The American Heritage® Dictionary of the English Language, 5th Edition.

  • noun Biology A close, prolonged association between two or more different organisms of different species that may, but does not necessarily, benefit each member.
  • noun A relationship of mutual benefit or dependence.

from The Century Dictionary.

  • noun Union for life of certain organisms, each of which is necessary to the other; an intimate vital consociation, or kind of consortism, differing in the degree and nature of the connection from inquilinity and parasitism, as in the case of the fungus and alga which together make up the so-called lichen, or of the fungus Mycorrhiza and various Cupuliferæ. See Lichenes, Mycorrhiza. Also called commensalism.

from the GNU version of the Collaborative International Dictionary of English.

  • noun (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 algæ and fungi to form lichens, and in the inclusion of algæ 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

  • noun A relationship of mutual benefit.
  • noun biology A close, prolonged association between two or more organisms of different species, regardless of benefit to the members.
  • noun The state of people living together in community.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • noun the relation between two different species of organisms that are interdependent; each gains benefits from the other


from The American Heritage® Dictionary of the English Language, 4th Edition

[Greek sumbiōsis, companionship, from sumbioun, to live together, from sumbios, living together : sun-, syn- + bios, life; see gwei- in Indo-European roots.]

from Wiktionary, Creative Commons Attribution/Share-Alike License

From Ancient Greek συμβίωσις (sumbiōsis, "living together").


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  • Why is the head definition for this word the opposite of the head definition for its adjective, symbiotic?

    June 22, 2008

  • Well spotted. Symbiotic has a more correct definition, while this one is more naïve and old-fashioned.

    June 22, 2008

  • No, I think it's the opposite. Symbiosis is the case where the whole is greater than the sum of its parts.

    June 22, 2008

  • No, seriously, jmp! The word symbiosis, for many reasons, is now used again in its literal meaning of "living together".

    I would not insist, but this is a field I know quite well - and I love. :-)

    June 22, 2008

  • Isn't it a feeding off of one another?

    August 9, 2008

  • Not necessarily, no. In fact if you have two organisms each consuming the other, it wouldn't be symbiotic because one or both would die as a result. But you might be thinking, for example, of those little fish that eat the parasites off sharks. The little fish get fed, and the shark gets rid of its parasites.

    August 9, 2008

  • Parasitic relationships are a kind of symbiosis, too - a kind with no reciprocity.

    August 9, 2008

  • Isn't reciprocity the essence of symbiosis? Parasitic and symbiotic relationships are mutually exclusive.

    August 9, 2008

  • As Wikipedia well explains, symbiosis is now generally used in a wider sense... Symbiosis sensu stricto can be more narrowly called mutualism.

    August 9, 2008

  • Bah, wikipedia. The work of ideologues and heathens!

    Actually that is interesting. Thank you, signor.

    August 9, 2008