tercentenary (adj.)

word-forming element meaning "thrice, three times," from Latin ter "thrice," from *tris-, from root of three. Compare Latin tertius "third."

1640s, "relation to or consisting of 100 years," from Latin centenarius "of a hundred, relating to a hundred," from centenai "a hundred each," from centum "hundred" (see hundred).

As a noun, c. 1600 as "period of 100 years;" 1788 as "a hundredth anniversary, commemoration or celebration of a hundredth anniversary." The usual British word in this sense for the American centennial.