<P> Symbiosis (from Greek συμβίωσις "living together", from σύν "together" and βίωσις "living") is any type of a close and long - term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasitic . The organisms, each termed a symbiont, may be of the same or of different species . In 1879, Heinrich Anton de Bary defined it as "the living together of unlike organisms". The term was subject to a century - long debate about whether it should specifically denote mutualism, as in lichens; biologists have now abandoned that restriction . </P> <P> Symbiosis can be obligatory, which means that one or both of the symbionts entirely depend on each other for survival, or facultative (optional) when they can generally live independently . </P>

Two organisms living together for their mutual benefit