In the sentence 'Washington is the capital of the United States,' the word 'capital' means:

CAPITAL is on the USCIS CIVICS VOCABULARY list. In civic/political context, CAPITAL means the CITY that serves as the seat of government. The capital of the United States is WASHINGTON, D.C. (District of Columbia). Sample reading test sentences: 'Washington is the capital of the United States.' 'The capital is in Washington.' 'The U.S. capital is in Washington.' Note the spelling distinction: CAPITAL (the city, the seat of government) vs CAPITOL (the building where Congress meets). These are HOMOPHONES (sound the same) but have different meanings. The U.S. CAPITOL is a specific building in Washington, D.C. — domed, located on Capitol Hill. The U.S. CAPITAL is the entire city of Washington, D.C. State capitals: each state has its own capital city — Sacramento (California), Albany (New York), Austin (Texas), Tallahassee (Florida), etc. ('capital' is on the USCIS list mainly in the federal context but applies to states too.) On the test, the word CAPITAL appears in sentences about Washington, D.C.