A parliamentarian is a specialist in parliamentary procedure.
The adjective parliamentarian refers to any person or thing especially associated with a parliament (see parliamentary system). A Parliamentarian (as an upper-cased noun) often labels a Member of Parliament or MP, especially one who is particularly adept in the chamber.
In the context of the political and constitutional and military history of England and Great Britain, the term parliamentarian and its derivatives often describe a party to the English Civil War of the mid-17th century -- the supporters of the rights of Parliament, as opposed to the Royalist defenders of monarchical right.
The Parliamentary cause could become fractionated. Thus, within the Parliamentary camp, religious Presbyterian partisans opposed Independents. On the political front, republicans or Commonwealth-men stood against constitutional monarchists, while Levellers and Diggers represented more radical tendencies.