Symbiosis

A term referring to a relationship in which an organism lives in close association with another species. It encompasses: mutualism, in which the relationship is beneficial to both partners; commensalism, in which the relationship is neither beneficial nor deleterious to either partner; and parasitism, in which the relationship is beneficial to one partner and deleterious to the other. The term symbiosis is also commonly used in the sense of mutualism, but the degree of benefit or harm to either partner is not always easy to assess. See also parasite.