commensalism (n.)

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.

word-forming element making nouns implying a practice, system, doctrine, etc., from French -isme or directly from Latin -isma, -ismus (source also of Italian, Spanish -ismo, Dutch, German -ismus), from Greek -ismos, noun ending signifying the practice or teaching of a thing, from the stem of verbs in -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached. For distinction of use, see -ity. The related Greek suffix -isma(t)- affects some forms.