1. Living together, social life.
2. Association of two different organisms (usually two plants, or an animal and a plant) which live attached to each other, or one as a tenant of the other, and contribute to each other’s support.

Also more widely, any intimate association of two or more different organisms, whether mutually beneficial or not.

3. The biological association of two or more species for their mutual benefit.
4. The mutual cooperation or interdependence of two people, as mother and infant, or husband and wife; sometimes used to denote excessive or pathological interdependence of two persons.

Here is more information and several illustrations about symbiosis.