Refers specifically to the legislative branch of the United States federal government, comprising the Senate and the House of Representatives.
The US Congress passed a new bill.
国会 is the generic term for a national parliament or congress, most commonly referring to Japan's National Diet. 米国議会 specifically denotes the US Congress.
Compound of 米国 (べいこく, 'United States') and 議会 (ぎかい, 'congress, parliament').