The Polish–Lithuanian Commonwealth, formally the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania, after 1791 the Commonwealth of Poland, was a dualistic state, a bi-confederation of Poland and Lithuania ruled by a common monarch, who was both the King of Poland and the Grand Duke of Lithuania. It was one of the largest and most populous countries of the 16th–17th century Europe. At its largest territorial extent, in the early 17th century, the Commonwealth spanned almost 400,000 square miles (1,000,000 km2) and sustained a multi-ethnic population of 11 million.
The Union possessed many features unique among contemporary states. Its political system was characterized by strict checks upon monarchical power. These checks were enacted by a legislature (sejm) controlled by the nobility (szlachta). This idiosyncratic system was a precursor to modern concepts of democracy,constitutional monarchy, and federation. Although the two component states of the Commonwealth were formally equal, Poland was the dominant partner in the union.
The official name of the state was The Kingdom of Poland and the Grand Duchy of Lithuania (Polish: Królestwo Polskie i Wielkie Księstwo Litewskie, Lithuanian: Lenkijos Karalystė ir Lietuvos Didžioji Kunigaikštystė, Latin: Regnum Poloniae Magnusque Ducatus Lithuaniae) and the Latin term was usually used in international treaties and diplomacy. In the 17th century and later it was also known as the Most Serene Commonwealth of Poland (Polish: Najjaśniejsza Rzeczpospolita Polska, Latin: Serenissima Res Publica Poloniae), the Commonwealth of the Polish Kingdom, or the Commonwealth of Poland. Its inhabitants referred to it in everyday speech as the "Rzeczpospolita" (Ruthenian: Рѣч Посполита Rech Pospolita, Lithuanian: Žečpospolita). Western Europeans often simply called it Poland and in most past and modern sources it is referred to as the Kingdom of Poland, or just Poland. The terms: the Commonwealth of Poland and the Commonwealth of Two Nations (Polish: Rzeczpospolita Obojga Narodów, Latin: Res Publica Utriusque Nationis) were used in the Reciprocal Guarantee of Two Nations. The English term 'Polish–Lithuanian Commonwealth' and German 'Polen-Litauen' are seen as renderings of the Commonwealth of Two Nations variant. Other names include the Republic of Nobles (Polish: Rzeczpospolita szlachecka) and the First Commonwealth (Polish: I Rzeczpospolita), the latter relatively common in Polish historiography.