What is the official term for a 250th anniversary, as used for America250?

Semiquincentennial is the official term for a 250th anniversary, as used for America250.

The word combines Latin-derived elements meaning half and five hundred, producing a term for half of a millennium. “America250” is the branding for the United States Semiquincentennial observance, centered on 2026, 250 years after 1776.

People sometimes call the event a quarter-millennium anniversary instead. That phrase is understandable, but semiquincentennial is the specialized anniversary noun used in formal descriptions and reference works. The term is less familiar than bicentennial, which means 200 years.

The United States previously marked its bicentennial in 1976, when celebrations, exhibitions, and a redesigned commemorative stamp series spread nationwide. The semiquincentennial follows that landmark by fifty years and links civic programs to the founding era.