Eastern Orthodoxy, otherwise known as Eastern Orthodox Christianity or Byzantine Christianity, is one of the three main branches of Chalcedonian Christianity, alongside Roman Catholicism and Protestantism. Like the Pentarchy of the first millennium, the mainstream Eastern Orthodox Church is organised into autocephalous churches independent from each other. In the 21st century, the number of mainstream autocephalous churches is seventeen; there also exist autocephalous churches unrecognized by those mainstream ones. Autocephalous churches choose their own primate. Autocephalous churches can have jurisdiction (authority) over other churches, some of which have the status of "autonomous" which means they have more autonomy than simple eparchies.

A Christian denomination is a distinct religious body within Christianity that comprises all church congregations of the same kind, identifiable by traits such as a name, particular history, organization, leadership, theological doctrine, worship style and, sometimes, a founder. It is a secular and neutral term, generally used to denote any established Christian church. Unlike a cult or sect, a denomination is usually seen as part of the Christian religious mainstream. Most Christian denominations refer to themselves as churches, whereas some newer ones tend to interchangeably use the terms churches, assemblies, fellowships, etc. Divisions between one group and another are defined by authority and doctrine; issues such as the nature of Jesus, the authority of apostolic succession, biblical hermeneutics, theology, ecclesiology, eschatology, and papal primacy may separate one denomination from another. Groups of denominations—often sharing broadly similar beliefs, practices, and historical ties—are sometimes known as "branches of Christianity". These branches differ in many ways, especially through differences in practices and belief.