a suffix, appearing in words of Latin origin, denoting action or condition, used in Latin and in English to form nouns from stems of Latin adjectives (communion; union), verbs (legion; opinion), and especially past participles (allusion; creation; fusion; notion; torsion).
Origin of -ion
< Latin -iōn- (stem of -iō) suffix forming nouns, especially on past participle stems; replacing Middle English -ioun < Anglo-French < Latin -iōn-
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suffix forming nouns
Word Origin for -ion
from Latin -iōn-, -io
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
suffix forming nouns of state, condition, or action from verbs, from Latin -ionem (nominative -io), sometimes via French -ion.
Online Etymology Dictionary, © 2010 Douglas Harper