1. A symbiosis (relation between two different species of organisms that are interdependent and each gains benefits from the other) characterized by the bodily union between the symbionts or one of two different organisms that live together.
2. Ordinarily symbiosis where the association is advantageous, or often necessary, to one or both, and not harmful to either.
3. When there is a bodily union in extreme cases so close that the two form practically a single body, as in the union of algae and fungi to form lichens, and in the inclusion of algae in radiolarians (any of various marine protozoans of the group Radiolaria, having rigid skeletons usually made of silica).