Decennary is from Latin decennārius, from decennium ('period of ten years'), from decem ('ten') plus annus ('year'). The ancestry passes through Latin decennium, Latin decem, and Latin annus. In present use as noun and adjective, it means "a period of ten years; relating to ten years", keeping the historical formation close to the word's ordinary semantic work. Related words include decennial, decennium, and decade. The surrounding word family keeps the same base visible in decennial, decennium, and decade, showing how the headword belongs to a larger cluster rather than standing alone.