For America's 250th anniversary, “semiquincentennial” describes 250 years.
The word combines “semi-,” meaning half, with “quincentennial,” relating to 500 years. In anniversary terminology, a semiquincentennial therefore marks half a millennium, or 250 years. It is one of several historical terms used for a quarter-millennium milestone.
America250 uses the term for the anniversary of American independence in 2026. The United States marked its bicentennial in 1976, exactly 200 years after the Declaration of Independence, so the 2026 observance is the next major national anniversary based on the Declaration’s adoption.
Terminology can vary. “Sestercentennial,” “bisesquicentennial,” and “quarter-millennial” have also appeared in historical or institutional usage, but “semiquincentennial” is the formal-sounding term most closely associated with the United States’ 250th anniversary. It should not be confused with “centennial,” which marks 100 years, or “bicentennial,” which marks 200 years.