Refers specifically to the bicameral legislature of the United States, consisting of the Senate and the House of Representatives.
The US Congress consists of the Senate and the House of Representatives.
The bill passed the US Congress.
国会 is the generic term for a national legislature, often used for Japan's Diet. アメリカ連邦議会 is the specific proper noun for the US Congress.
Compound of アメリカ (America) + 連邦 (federation) + 議会 (parliament, congress). A straightforward calque for the United States Congress.