cen·ten·ar·y / senˈtenərē; ˈsentnˌerē/ chiefly Brit. • n. (pl. -ar·ies) the hundredth anniversary of a significant event; a centennial. ∎  a celebration of such an anniversary.• adj. of or relating to a hundredth anniversary; centennial.

centenary adj. of a hundred years XVII; sb. † 100 pounds XVI; century XVII; centennial anniversary XVIII. — L. centēnārius, containing a hundred, f. centēnī hundred each, f. centum HUNDRED; see -ARY.
So centenarian 100 years old. XIX. centennial of 100 years. XVIII. f. L. centum, after biennial. centesimal hundredth (part). XVII. f. L. centēsimus hundredth.