abbreviation for rex or regina, 'king' or 'queen'. The initial letter is used in proceedings, especially English criminal proceedings, to indicate that the Crown is the plaintiff or, more usually, prosecutor: R v. Haddock. In Scotland, prosecutions run in the name of Her (or His) Majesty's Advocate, hence cases are cited HMA v . McFable.
Collins dictionary of law. W. J. Stewart. 2001.