Definition of Commensal

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Commensal: 1. Living in a relationship in which one organism derives food or other benefits from another organism without hurting or helping it. Commensal bacteria are part of the normal flora in the mouth.
2. An intimate relationship. The Norway rat, roof rat, and house mouse are considered commensal rodents because of their intimate relationships with humans.

From the Latin prefix com- (with, together, jointly) + the Latin adjective mensalis (of the table). Commensal people eat together at the same table.