Calvinism (also called the Reformed tradition, Reformed Protestantism, Reformed Christianity, or simply Reformed) is a major branch of Protestantism that follows the theological tradition and forms of Christian practice set down by John Calvin and other Reformation-era theologians. It emphasizes the sovereignty of God and the authority of the Bible.

Calvinists broke from the Roman Catholic Church in the 16th century. Calvinists differ from Lutherans (another major branch of the Reformation) on the spiritual real presence of Christ in the Lord's Supper, theories of worship, the purpose and meaning of baptism, and the use of God's law for believers, among other points. The label Calvinism can be misleading, because the religious tradition it denotes has always been diverse, with a wide range of influences rather than a single founder; however, almost all of them drew heavily from the writings of Augustine of Hippo twelve hundred years prior to the Reformation.

The namesake and founder of the movement, French reformer John Calvin, embraced Protestant beliefs in the late 1520s or early 1530s, as the earliest notions of later Reformed tradition were already espoused by Huldrych Zwingli. The movement was first called Calvinism in the early 1550s by Lutherans who opposed it. Many in the tradition find it either a nondescript or inappropriate term and prefer the term Reformed. The most important Reformed theologians include Calvin, Zwingli, Martin Bucer, William Farel, Heinrich Bullinger, Thomas Cranmer, Nicholas Ridley, Peter Martyr Vermigli, Theodore Beza, and John Knox. In the twentieth century, Abraham Kuyper, Herman Bavinck, B. B. Warfield, J. Gresham Machen, Louis Berkhof, Karl Barth, Martyn Lloyd-Jones, Cornelius Van Til, R. C. Sproul, and J. I. Packer were influential. Contemporary Reformed theologians include Albert Mohler, John MacArthur, Tim Keller, John Piper, Joel Beeke, and Michael Horton.

The Reformed tradition is largely represented by the Continental Reformed, Presbyterian, Evangelical Anglican, Congregationalist, and Reformed Baptist denominations. Several forms of ecclesiastical polity are exercised by a group of Reformed churches, including presbyterian, congregationalist, and some episcopal. The biggest Reformed association is the World Communion of Reformed Churches, with more than 100 million members in 211 member denominations around the world. More conservative Reformed federations include the World Reformed Fellowship and the International Conference of Reformed Churches.