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Rev. Leo A. Walsh, S.T.D. Answers:
An ecumenical council is when the entire Church, through the bishops, gathers together to address one or more issues vital to the life of the universal Church at a given period of time. The first seven councils of the Church are generally accepted and called the “Ecumenical Councils.” The First Seven Ecumenical Councils, as commonly understood, are:
First Council of Nicaea (325)
First Council of Constantinople (381)
Council of Ephesus (431)
Council of Chalcedon (451)
Second Council of Constantinople (553)
Third Council of Constantinople (680)
Second Council of Nicaea (787)
However, the term has also sometimes been used within the Catholic household of the faith to denote councils of all the Catholic Bishops. These are sometimes also called “Western Councils” and there have been 27 of them in all.
One could argue that the Second Vatican Council could be called an “ecumenical council” in that, although it was primarily concerned with matters pertaining to the Catholic Church (both Latin and Eastern Catholics), for the first time in over a thousand years there were a number of non-voting representatives from the Orthodox Churches and Protestant denominations.