Bicentennial means the day or year exactly 200 years after an important event — the same idea as bicentenary. Americans use this word more. They use it as an adjective (a bicentennial parade) and as a noun (the bicentennial). The big US example many textbooks mention is 1976, two hundred years after 1776. British writing still often prefers bicentenary for the noun. Though actually, both words are built the same way: bi- (two) plus a “hundred years” piece. If you only remember one Chinese hook, 兩百週年 covers both. It is a ceremony-and-history word, not an everyday one. Do not use it for a person’s 200th birthday unless you are talking about a famous birth long ago, or a city, a school, a country.