Is the senate the upper house or lower house?

The lower house is usually the House of Representatives. When a bill is first introduced, read by everyone in the house and placed on the agenda for the next session, the bill is then debated, scrutinized and requesting for amendments in the lower house.

When the bill is accepted through the House of Representatives (lower house), the bill is then passed on to the upper house (Senate House) and further debated upon. If the majority of seats in the Senate House belongs to the Government Party, the bill can be easily passed through. If the majority of seats belongs to the Opposition, the passing of the bill might get complicated. The debating of passing the bill is pursued and voted upon. If the bill needs amendments, it'll be passed back to the lower house and the entire process is repeated until the bill is passed to both the houses (bicameral legislature)

Easy to say, the lower house is where the Prime Minister lives and where most the heat begins, and the upper house has the authority to refuse the bill and knock it back.