A senate is a deliberative body, often the upper house or chamber of a legislature. The word senate is derived from the Latin word senex (old man), via the Latin word senatus (senate). The Latin word senator has been adopted by English with no change in spelling. Its meaning comes from a very ancient form of even simple social organisation in which the decisional power is reserved to the eldest men. For the same reason, the word senate is correctly used when referring to any powerful authority characteristically composed by the eldest members of a community, as a deliberative body of a faculty in an institution of higher learning is often called a senate. The original senate was the Roman Senate.

Modern democratic states with bicameral parliamentary systems are sometimes organised with a senate, often distinguished by an ordinary parallel lower house (usually called the 'House of Representatives', 'Chamber of Deputies' or 'National Assembly'), by electoral rules (minimum age required for voters and candidates, proportional or majoritarian/plurality system, electoral basis = collegium). Typically, the senate is referred to as the upper house and has a smaller membership than the lower house.

An example of this is the United States Senate where the number of seats is fixed at two per state, regardless of size.

In a federal system, the senate often serves a balancing effect by giving a larger share of power to regions and groups which would otherwise be overwhelmed in a purely representative system. In the legislatures of U.S. states, Senates were also used for this purpose until the 1963 case of Baker v. Carr, in which the Supreme Court of the United States ruled that state legislatures must apportion seats in both houses according to population. However, there are still typically fewer members of a state Senate than there are members of the lower house.

In the United States, each of its member states has a Senate and a lower chamber, known variously as the House of Representatives, House of Delegates, General Assembly or Assembly, except for the state of Nebraska, where the Senate is the only body of a unicameral legislature.

Examples of senates in other states include the Australian Senate, the Canadian Senate, the French Senate, the Irish Senate (Seanad Éireann), the Italian Senate, the Philippine Senate, and the Senate of Poland.

A senate can also be the name of an executive branch of government. Until 1919, the Senate of Finland was the executive branch and supreme court. In the German city Länder or states, of Berlin, Bremen, and Hamburg, the executive branch of government is called the Senate, with Senators holding ministerial portfolios.