From Wiktionary, the free dictionary
Borrowed from Latin -ennium, from -ennis (annus + -is) + -ium.
-ennio m (noun-forming suffix, plural -enni)
- (no longer productive) forms nouns from numbers, meaning "period of ~ years"
- mille (“a thousand”) + -ennio → millennio (“millennium”)
- dieci (“ten”) + -ennio → decennio (“decade”)
- trenta (“thirty”) + -ennio → trentennio (“a period of thirty years”)