The term "Polish-Lithuanian" primarily refers to the historical union between the Kingdom of Poland and the Grand Duchy of Lithuania, which formed the Polish-Lithuanian Commonwealth in 1569. This political entity lasted until the late 18th century and was notable for its unique system of governance, which included a shared monarch and a parliament known as the Sejm. The Commonwealth was one of the largest and most populous countries in Europe at the time, promoting a diverse culture and religious tolerance. It played a significant role in European politics and influenced the development of both Poland and Lithuania as modern nations.