Real Dictionary

Uncle Sam
proper noun: Some people refer to the United States of America or its government as Uncle Sam.

Uncle Sam in American English
the U.S. (government or people), personified as a tall, spare man with chin whiskers, dressed in a red, white, and blue costume of swallow-tailed coat, striped trousers, and tall hat with a band of stars
noun: a personification of the government or people of the U.S.: represented as a tall, lean man with white chin whiskers, wearing a blue tailcoat, red-and-white-striped trousers, and a top hat with a band of stars

Uncle Sam in British English
noun: a personification of the government of the United States