A parliament is a legislature, the law-making institution of a government. Generally, a modern parliament has three functions: representing the electorate, making laws, and overseeing the executive government via hearings and inquiries. The term is similar to the idea of a senate, synod or congress and is commonly used in countries that are current or former monarchies. Some contexts restrict the use of the word parliament to parliamentary systems, although it is also used to describe the legislature in some presidential systems (e.g., the Parliament of Ghana), even where it is not in the official name.

Historically, parliaments included various kinds of deliberative, consultative, and judicial assemblies. What is considered to be the first modern parliament was the Cortes of León, held in the Kingdom of León in 1188. According to the UNESCO, the Decreta of Leon of 1188 is the oldest documentary manifestation of the European parliamentary system. UNESCO granted the 1188 Cortes of Alfonso IX the title of "Memory of the World" and the city of Leon has been recognized as the "Cradle of Parliamentarism".

The English term is derived from Anglo-Norman and dates to the 14th century, coming from the 11th century Old French word parlement 'discussion, discourse', from parler, 'to talk'. The meaning evolved over time, originally referring to any discussion, conversation, or negotiation through various kinds of deliberative or judicial groups, often summoned by a monarch. By the 15th century, in Britain, it had come to specifically mean the legislature.

Since ancient times, when societies were tribal, there were councils or a headman whose decisions were assessed by village elders. This is called tribalism. Some scholars suggest that in ancient Mesopotamia there was a primitive democratic government where the kings were assessed by council. The same has been said about ancient India, where some form of deliberative assemblies existed, and therefore there was some form of democracy. However, these claims are not accepted by other scholars, who see these forms of government as oligarchies.

Ancient Athens was the cradle of democracy. The Athenian assembly (ἐκκλησία, ekklesia) was the most important institution, and every free male citizen could take part in the discussions. Slaves and women could not. However, Athenian democracy was not representative, but rather direct, and therefore the ekklesia was different from the parliamentary system.

The Roman Republic had legislative assemblies, who had the final say regarding the election of magistrates, the enactment of new statutes, the carrying out of capital punishment, the declaration of war and peace, and the creation (or dissolution) of alliances. The Roman Senate controlled money, administration, and the details of foreign policy.

Some Muslim scholars argue that the Islamic shura (a method of taking decisions in Islamic societies) is analogous to the parliament. However, other scholars (notably from Hizb ut-Tahrir) highlight what they consider fundamental differences between the shura system and the parliamentary system.

England has long had a tradition of a body of men who would assist and advise the king on important matters. Under the Anglo-Saxon kings, there was an advisory council, the Witenagemot. The name derives from the Old English ƿitena ȝemōt, or witena gemōt, for "meeting of wise men". The first recorded act of a witenagemot was the law code issued by King Æthelberht of Kent around 600, the earliest document which survives in sustained Old English prose; however, the Witan was certainly in existence long before then. The Witan, along with the folkmoots (local assemblies), is an important ancestor of the modern English parliament.