commensal (adj.)

late 14c., "eating together at the same table, sharing the table with the host," from Medieval Latin commensalis, from com "with, together" (see com-) + mensa (genitive mensalis) "table" (see mesa). 

As a noun, "one who eats at the same table" (as another), early 15c. Biological sense "one of two animals or plants which live together but neither at the expense of the other" is attested from 1870.

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