Symbiosis (from Ancient Greek σύν "together" and βίωσις "living") is close and often long-term interaction between different biological species. In 1877, Bennett used the word symbiosis (which previously had been used of people living together in community) to describe the mutualistic relationship in lichens. In 1879, by theGerman mycologist Heinrich Anton de Bary, defined it as "the living together of unlike organisms."]
The definition of symbiosis is controversial among scientists. Some believe symbiosis should only refer to persistent mutualisms, while others believe it should apply to any types of persistent biological interactions (i.e.mutualistic, commensalistic, or parasitic)]
Some symbiotic relationships are obligate, meaning that both symbionts entirely depend on each other for survival. For example, many lichens consist of fungal and photosynthetic symbionts that cannot live on their own. Others are facultative, meaning that they can, but do not have to live with the other organism.
Symbiotic relationships include those associations in which one organism lives on another (ectosymbiosis, such as mistletoe), or where one partner lives inside the other (endosymbiosis, such as lactobacilli and other bacteria in humans or zooxanthelles in corals)