Ecumenical councils are assemblies of bishops whose doctrinal definitions and canons have been received by the Church as authoritative for the universal Christian faith.
The seven councils
Eastern Orthodox tradition commonly numbers seven: Nicaea I (325), Constantinople I (381), Ephesus (431), Chalcedon (451), Constantinople II (553), Constantinople III (680–681), and Nicaea II (787). They addressed the Trinity, the person and natures of Christ, the title Theotokos, and the proper veneration of icons.
Reception
A council’s authority is not merely political or numerical. Its teaching is received as faithful to Scripture and apostolic Tradition in the continuing life of the Church.