Where does “decennium” come from?
decennium (English) comes from English decennial, from Latin decennialis, from Latin -ennium, from Latin -ennis, from Latin annus, from Proto-Italic atnos, from Proto-Indo-European h₂et-no-, from Proto-Indo-European h₂et- — grass.
decennium (English): A period of ten years
Ancestry of “decennium”, step by step