decade (n.)

mid-15c., "ten parts" (of anything; originally in reference to the divisions of Livy's history), from Old French décade (14c.), from Late Latin decadem (nominative decas), from Greek dekas (genitive dekados) "group of ten," from deka "ten" (from PIE root *dekm- "ten"). Meaning "period of ten consecutive years" is 1590s in English. Related: Decadal; decadary.