senate 名詞 （アメリカなどの）上院 ˈsɛnɪt the smaller and more senior of the two groups that make up the legislative body of the government in the US and some other countries
House of Representatives =the larger of the two elected groups of the US Congress Congress =the group of people who make the laws
The Legislative Branch enacts legislation, confirms or rejects Presidential appointments, and has the authority to declare war. This branch includes Congress (the Senate and House of Representatives) and several agencies that provide support services to Congress.
「US Congress」が２つあって、 その中の人数が少ないのが上院、多いのが下院