Separation of church and state is a political and legal doctrine that government and religious institutions are to be kept separate and independent from each other. The term most often refers to the combination of two principles: secularity of government and freedom of religious exercise.

Reflecting a concept often credited in its original form to the English political philosopher John Locke , the phrase separation of church and state is generally traced to the letter written by Thomas Jefferson in 1802 to the Danbury Baptists, in which he referred to the First Amendment to the United States Constitution as creating a "wall of separation" between church and state. The phrase was quoted by the United States Supreme Court first in 1878, and then in a series of cases starting in 1947. This led to increased popular and political discussion of the concept.

The concept has since been adopted in a number of countries, to varying degrees depending on the applicable legal structures and prevalent views toward the proper role of religion in society. A similar principle of laïcité has been applied in France and Turkey, while some socially secularized countries such as Norway have maintained constitutional recognition of an official state religion. The concept parallels various other international social and political ideas, including secularism, disestablishment, religious liberty, and religious pluralism.