English

decadal

From decade + -al.

A compound of decade + -al

Proto-Indo-European *déḱm̥ ‘ten’

…

Ancient Greek δεκάς ‘the number ten’

Late Latin decās ‘(set of) ten’

Old French decade

Middle English decade

Today it means

adj Of or related to the number ten.

Language periods are conventional ranges, not dates for this word.