A parliament is a legislature. More specifically, "parliament" may refer only to a democratic government's legislature. The term is derived from the French parlement, the action of parler ("to speak"): a parlement is a discussion. In around 1300 CE the term came to mean a meeting at which such a discussion took place. It acquired its modern meaning, in the mid-14th century, as it came to be used for the body of people (in an institutional sense) who would meet to discuss matters of state.
Generally, a parliament has three functions: representation, legislation and parliamentary control (i.e., hearings, inquiries).
Legislatures called parliaments operate under a parliamentary system of government in which the executive is constitutionally answerable to the parliament. Parliaments usually consist of chambers or houses, and are usually either bicameral or unicameral although more complex models exist, or have existed (see Tricameralism). The Union has a unicameral, but not parliamentary government as the Assembly and its staff also act as the executive.
A nation's prime minister (PM) is almost always the leader of the majority party in the lower house of parliament, but only holds his or her office as long as the "confidence of the house" is maintained. If members of the lower house lose faith in the leader for whatever reason, they can call a vote of no confidence and force the PM to resign.
This can be particularly dangerous to a government when the distribution of seats among different parties is relatively even, in which case a new election is often called shortly thereafter. However, in case of general discontent with the head of government, his replacement can be made very smoothly without all the complications that it represents in the case of a presidential system.
The parliamentary system can be contrasted with a presidential system, on the model of the United States' congressional system, which operate under a stricter separation of powers whereby the executive does not form part of, nor is appointed by, the parliamentary or legislative body. In such a system, congresses do not select or dismiss heads of governments, and governments cannot request an early dissolution as may be the case for parliaments. Some states have a semi-presidential system which falls between parliamentary and congressional systems, combining a powerful head of state (President) with a head of government (PM) responsible to parliament.