Bimillennium refers to a period of two thousand years. There are several synonyms for this word that can be used in different contexts. One synonym is "biennial," which refers to something that happens every two years. Another synonym is "millennium," which refers to a period of one thousand years. "Duo-century" and "double millennium" are also used interchangeably with bimillennium. In academic contexts, the term "bi-millennial" is also used to describe a period of two thousand years. Regardless of the term used, these words all convey the idea of a span of time that is significant and profound.