1. any relationship in which two species live together in close association, especially one in which both species benefit (see mutualism). For example, in tropical Amazonia, a species of ant lives on a particular tree species that it uses for food and shelter, at the same time removing lichen and other parasites that might harm the tree. Also called biological symbiosis.

2. the stage in infantile development when the infant’s dependence is total and he or she is neither biologically nor psychologically separated from the mother. See also separation–individuation. [proposed by Hungarian-born U.S. child psychoanalyst Margaret Schönberger Mahler (1897–1985)]

3. a mutually reinforcing relationship in which one individual is overdependent on another to satisfy needs. Such a relationship hampers the development or independence of both individuals and usually results in dysfunction when the dominant individual is unwilling to provide for the dependent individual. Also called symbiotic relationship. —symbiotic adj.