Symbiosis (from Greek συμβίωσις, symbíōsis, "living together", from σύν, sýn, "together", and βίωσις, bíōsis, "living") is any type of a close and long-term biological interaction between two biological organisms of different species, termed symbionts, be it mutualistic, commensalistic, or parasitic.

Symbiosis (pl. symbioses) means living together. It describes close and long-term relationships between different species. The term was used by Anton de Bary in 1879, as "the living together of unlike organisms". A symbiont is an organism living in a relationship with another species in which one or both get benefits.

Symbiosis (plural: "symbioses") is the close, interactive association (living together) of members of two or more species. The term symbiosis was coined by the German botanist Anton de Bary in 1879 from the Greek symbioun (to live together), from the prefix sym (sum, together, or together with) and the word bios (life).

Symbiosis, any of several living arrangements between members of two different species, including mutualism, commensalism, and parasitism. Both positive (beneficial) and negative (unfavorable to harmful) associations are therefore included, and the members are called symbionts.

Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms.