Symbiosis

The term symbiosis (from the Greek: συμ, sym, "with"; and βίοσίς, biosis, "living") commonly describes close and often long-term interactions between different biological species. The term was first used in 1879 by the German mycologist, Heinrich Anton de Bary, who defined it as: "the living together of unlike organisms".
The definition of symbiosis is in flux and the term has been applied to a wide range of biological interactions. The symbiotic relationship may be categorized as being mutualistic, parasitic, or commensal in nature Others define it more narrowly, as only those relationships from which both organisms benefit, in which case it would be synonymous with mutualism.
Symbiotic relationships included those associations in which one organisms lives on another (ectosymbiosis, such as mistletoe), or where one partner lives inside another (endosymbiosis, such as lactobacilli and other bacteria in humans or zooxanthelles in corals). Symbiotic relationships may be either obligate, i.e., necessary to the survival of at least one of the organisms involved, or facultative, where the relationship is beneficial but not essential to survival of the organisms.