quincentenary

before vowels quinqu-, word-forming element from classical Latin meaning "five, consisting of or having five," from Latin quinque "five" (by assimilation from PIE root *penkwe- "five").

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.