The Deluge was a series of mid-17th-century military campaigns in the Polish–Lithuanian Commonwealth. In a wider sense, it applies to the period between the Khmelnytsky Uprising of 1648 and the Truce of Andrusovo in 1667, comprising the Polish theatres of the Russo-Polish and Second Northern Wars. In a stricter sense, the term refers to the Swedish invasion and occupation of the Commonwealth as a theatre of the Second Northern War (1655–1660) only; in Poland and Lithuania this period is called the Swedish Deluge, or less commonly the Russo–Swedish Deluge due to the simultaneous Russo-Polish War. The term "deluge" was popularized by Henryk Sienkiewicz in his novel The Deluge (1886).

The Polish–Lithuanian Commonwealth, formally known as the Kingdom of Poland and the Grand Duchy of Lithuania, or simply Poland–Lithuania, was a bi-confederal state, sometimes called a federation, of Poland and Lithuania ruled by a common monarch in real union, who was both King of Poland and Grand Duke of Lithuania. It was one of the largest and most populous countries of 16th- to 17th-century Europe. At its largest territorial extent, in the early 17th century, the Commonwealth covered almost 1,000,000 km2 (400,000 sq mi) and as of 1618 sustained a multi-ethnic population of almost 12 million. Polish and Latin were the two co-official languages, and Roman Catholicism served as the state religion.