Washington Dc
In short
The capital city of the United States and its only federal district. It is located on the Potomac River, bordered by Maryland and Virginia.
In plain words
It is a special area that does not belong to any single state. It serves as the main centre for the American government and its many national institutions.
A simple example
The White House and the US Capitol building are both located within this district.
Why it matters
It is the seat of power for the US federal government. Most major political decisions and international diplomacy happen here.
Easy to mix up
People often confuse it with Washington State, which is located on the west coast of the country.