-ison

Forming nouns.

Old French ‑a(i)son or ‑e(i)son, from Latin ‑atio(n)‑, ‑etio(n)‑, or ‑itio(n)‑.

A small number of such nouns exists, of which the most common are comparison, jettison (usually now a verb), and venison. See also ‑ation, which is now the active suffix from the same source.