a suffix appearing in loanwords from Greek, where it was used to form from verbs abstract nouns of action, process, state, condition, etc.: thesis; aphesis.
Origin of -sis
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
suffix in Greek-derived nouns denoting action, process, state, condition, from Greek -sis, which is identical in meaning with Latin -entia, English -ing (1).
Online Etymology Dictionary, © 2010 Douglas Harper