a pact, with the force of international law, concluded between the ecclesiastical authority and the secular authority on matters of mutual concern; most especially a pact between the pope, as head of the Roman Catholic (Roman Catholicism) church (church and state), and a temporal head of state for the regulation of ecclesiastical affairs in the territory of the latter. Matters often dealt with in concordats include: the rights and liberties of the church; the creation and suppression of dioceses and parishes; the appointment of bishops, pastors, and military chaplains, sometimes with provision for their support; ecclesiastical immunities (e.g., exemption from military service); church property; questions relating to marriage; and religious education.

The earliest concordats were associated with the Investiture Controversy that profoundly agitated Christian Europe in the 11th and 12th centuries; the most important was the Concordat of Worms (Worms, Concordat of) (1122) between Pope Calixtus II and Emperor Henry V. In the 19th century a long list of concordats was concluded, of which a good number remain in force. The first in date and importance was that of 1801 (1801, Concordat of), concluded for France by Napoleon and Pope Pius VII after laborious negotiations; it was denounced by the French government in 1905 with its law of separation of church and state. During the 20th century many new concordats were made, including the Lateran Treaty (1929–85) with Italy, which recognized papal sovereignty over the Vatican city-state.