(symbiotic) association of two different types of living organisms that is often. but not always. beneficial to each.
Symbiosis (Ancient Greek συμβίωσις symbíōsis: living with, companionship < σύν sýn: together; and βίωσις bíōsis: living) is any type of a close and long-term biological interaction, between two organisms of different species. The two organisms, termed symbionts, can be either in a mutualistic, a commensalistic, or a parasitic relationship. In 1879, Heinrich Anton de Bary defined symbiosis as "the living together of unlike organisms".
The term is sometimes more exclusively used in a restricted, mutualistic sense, where both symbionts contribute to each other's subsistence.
Symbiosis can be obligatory, which means that one, or both of the symbionts depend on each other for survival, or facultative (optional), when they can also subsist independently.
Symbiosis is also classified by physical attachment. Symbionts forming a single body live in conjunctive symbiosis, while all other arrangements are called disjunctive symbiosis. When one organism lives on the surface of another, such as head lice on humans, it is called ectosymbiosis; when one partner lives inside the tissues of another, such as Symbiodinium within coral, it is termed endosymbiosis.