Symbiosis can be obligatory, which means that one or both of the symbionts entirely depend on each other for survival, or facultative (optional) when they can generally live independently.
Symbiosis is also classified by physical attachment; symbiosis in which the organisms have bodily union is called conjunctive symbiosis, and symbiosis in which they are not in union is called disjunctive symbiosis. When one organism lives on another such as mistletoe, it is called ectosymbiosis, or endosymbiosis when one partner lives inside the tissues of another, as in Symbiodinium in corals.
The definition of symbiosis was a matter of debate for 130 years. In 1877, Albert Bernhard Frank used the term symbiosis to describe the mutualistic relationship in lichens. In 1879, the German mycologist Heinrich Anton de Bary defined it as "the living together of unlike organisms." The definition has varied among scientists with some advocating that it should only refer to persistent mutualisms, while others thought it should apply to all persistent biological interactions, in other words mutualisms, commensalism, or parasitism, but excluding brief interactions such as predation. Current biology and ecology textbooks use the latter "de Bary" definition, or an even broader one where symbiosis means all interspecific interactions; the restrictive definition where symbiosis means only mutualism is no longer used.
In 1949, Edward Haskell (1949) proposed an integrative approach, proposing a classification of "co-actions", later adopted by biologists as "interactions".
Biological interactions can involve individuals of the same species (intraspecific interactions) or individuals of different species (interspecific interactions). These can be further classified by either the mechanism of the interaction or the strength, duration and direction of their effects.