A plaintiff (Π in legal shorthand) is the party who initiates a lawsuit (also known as an action) before a court.
In most English-speaking jurisdictions, including Hong Kong, Nigeria, Australia (except in federal jurisdiction), Canada, the United States, Northern Ireland and the Republic of Ireland, the legal term "plaintiff" is used as a general term for the party taking action in a civil case.
In most common-law jurisdictions, the term "claimant" used in England and Wales since 1999 (see below) is used only in specific, often non-judicial contexts.
In particular, in American usage, terms such as "claimant" and "claim form" are limited to extrajudicial process in insurance and administrative law.
In England and Wales, the term "claimant" replaced "plaintiff" after the Civil Procedure Rules came into force on 26 April 1999.