quincentennium

Formed from the compounding of quin- (“five”) +‎ centennium (“period of a hundred years”); compare the earlier quincentennial. Etymologically regular formation (stemming from the Latin quīnque, “five”) would produce *quinquecentennium.

quincentennium (plural quincentennia)

1. (rare) A period of five hundred years; half a millennium.
   • 1972, Scottish International V, page 11:

     And Mr John Graham, editor of The New Shetlander and one of the most articulate ideologues of the 1967 revolution, reinterpreted the problem of “Social Change during the Quincentennium” (1469–1969) accordingly.

   • 1972, Wilfred Jenks, “Realism and Magnanimity in the Law of Nations” in The American Journal of International Law LXVI, № 4 (September 1972), page 316:

     In more leisurely days there were Egyptian millenia, [sic] Greek and Roman quincentennia, and Spanish, French, and British centuries.