In ecology, Commensalism is a class of relationship between two organisms where one organism benefits but the other is unaffected. There are two other types of association: mutualism (where both organisms benefit) and parasitism (one organism benefits and the other one is harmed).

Commensalism derives from the English word commensal, meaning "sharing of food" in human social interaction, which in turn derives from the Latin cum mensa, meaning "sharing a table".

An example of commensalism would be birds following army ant raids on a forest floor. As the army ant colony travels on the forest floor, they stir up various flying insect species. As the insects flee from the army ants, the birds following the ants catch the fleeing insects. In this way, the army ants and the birds are in a commensal relationship because the birds benefit while the army ants are unaffected. The D. folliculorum mites living in human eyelash follicles have a similar relationship with their hosts. Orchids and mosses are plants that can have a commensal relationship with trees. The plants grow on the trunks or branches of trees. They get the light they need as well as nutrients that run down along the tree. As long as these plants do not grow too heavy, the tree is not affected.

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• Remora - phoretic fishes that may also eat the scraps off their hosts, though they may also be mutualistic cleaners

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