SPQR is an acronym of a Latin phrase, Senātus Populusque Rōmānus (“The Senate and People of Rome“; Classical Latin: [sɛˈnaː.tʊs po:pʊˈlʊs.kᶣɛ roːˈmaː.nʊs]), referring to the government of the ancient Roman Republic, and used as an official emblem of the modern-day comune (municipality) of Rome. It appears on coins, at the end of documents made public by inscription in stone or metal, in dedications of monuments and public works, and was emblazoned on the standards of the Roman legions.

The phrase appears countless times in Roman political, legal and historical literature, including the speeches of Cicero and the Ab urbe condita libri (“Books from the Founding of the City”) of Titus Livius (Livy). wikipedia.org