Unappropriated Duty stamp - popular name for British fiscal stamps which left space in the design for the overprint of the specific purpose or duty
Unemployed Intellectual stamp - a postage stamp whose revenue helped support unemployed intellectuals and artists. This stamp was used in France (1935-1940) for intellectuals and in Hungary (1940) for artists
Unissued stamp - a stamp officially prepared for postal use but never issued due to, for example, an error in design or inscription, sudden change in political regime, or postal rates
United Postal Union - an institution originally known as the General Postal Union (established in 1874 and renamed in 1878). The Universal Postal Union coordinates postal policies among member nations.
UPU - abbreviation for the Universal Postal Union. Originally known as the General Postal Union (established in 1874 and renamed in 1878), the Universal Postal Union coordinates postal policies among member nations.
USPS - abbreviation for the U.S. Postal Service. The U.S. Post Office Department was reorganized in 1971. As a part of the transformation, it was renamed the 'U.S. Postal Service'.