A quincentennial is a 500th anniversary or a celebration of it. The word can also be used to describe anything relating to a 500th anniversary, such as a quincentennial parade or collectible coin.
Quin- comes from the Latin quinque, meaning "five." Centennial comes from the Latin cent, meaning "one hundred." And the English suffix ennial means "some amount of years." So put together, quincentennial means "a period of five hundred years." It's most often used for anniversaries, like the founding of a country, for example. (The United States of America won't have its quincentennial until 2276!) Another term for this concept is quincentenary.