U.S.

1. Uncle Sam
2. Uniform System (of lens aperture)
3. United States
4. United States highway

The noun has 2 meanings:

Meaning #1: the executive and legislative and judicial branches of the federal government of the United States
  Synonyms: United States government, United States, U.S. government

Meaning #2: North American republic containing 50 states - 48 conterminous states in North America plus Alaska in northwest North America and the Hawaiian Islands in the Pacific Ocean; achieved independence in 1776
  Synonyms: United States, United States of America, America, US, USA, U.S.A.