'Decade' is Greek for 'a group of ten' — from PIE *dekm. It meant any set of ten before meaning years.
A period of ten years; a group or set of ten.
From Old French 'decade,' from Late Latin 'decas' (genitive 'decadis'), from Greek 'dekás' (δεκάς, a group of ten), from 'déka' (δέκα, ten). The PIE root is *deḱm̥ (ten), one of the most stable and well-attested numerals in the proto-language, cognate with virtually every Indo-European branch. From *deḱm̥ came Latin 'decem' (ten), Old English 'tēon' (ten), Sanskrit
December was originally the tenth month of the Roman calendar (which began in March), and its name from Latin 'decem' (ten) still reflects this. When January and February were added to the beginning of the calendar, December became the twelfth month but kept its 'tenth month' name. The same mismatch affects September (7th → 9th), October