• commensal •
Pronunciation: kê-men-sêl • Hear it!
Part of Speech: Adjective
Meaning: 1. Eating at the same table with someone, sharing a table. 2. Pertaining to a symbiotic relationship in which one species benefits but not the other.
Notes: Symbiotic relationships come in three main types: (1) mutualism (both species benefit), (2) commensalism (only one species benefits), and (3) parasitism (one benefits, the other is harmed). The noun from this adjective, as you can see, is commensalism. The adverb is commensally with two Ls. The adjective may be used as a noun, as in, "Henry is a commensal of mine."
In Play: 
The birds in the illustration are in a commensal relationship with the cows: they eat insects stirred up by the grazing of the cows. However, the original meaning of today's Good Word was the first one above, though it is rarely used today: "Our relationship at college was purely commensal; we enjoyed conversations over a good meal." Still, we can stretch the meaning of this word to other relationships: "Friedrich, your relation to this company is not commensal: you have to do some work in return for you salary."
Word History: Today's word was taken from Latin commensalis "sharing a table", derived from com "(together) with" + mensa "table". Com- is the combining form of cum "together" with", and derives from PIE kom "by, with". Kom became the collective prefix ge- in Old English and remains in Modern German in such words as Gebirge "mountains", Gemeinde "community", Gefuge "structure". Mensa seems to be the feminine past participle of Latin metior "to measure". A possible semantic connection could be that tables in the stalls of the markets were of a standard length and were used for measuring, but that is only my guess. This word apparently has a Fickle N, not found in meter or measure. (Today's Good Word was recommended by Jeremy Busch, a Grand Panjandrum in the Agora, whose relationship with this website is mutualistic.)
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