The definition of commensalism is a relationship between two organisms where one is helped and the other is unaffected.(noun)
An example of commensalism is when a bird lives in a tree; the bird is helped by living in the tree because it has a place to build its nest and it is high up so protected from predators, but the tree is not affected.
See commensalism in Webster's New World College Dictionary
See commensalism in American Heritage Dictionary 4
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