The American Church Dictionary and Cycopedia
From a Greek word meaning general or universal. The name is given to certain councils composed of Bishops and other ecclesiastics from the whole Church. A Council to be ecumenical must meet three requirements: (1) It must be called of the whole Catholic Church; (2) it must be left perfectly free, and (3) it must be one whose decrees and definitions were subsequently accepted by the whole Church. It is commonly believed that there have been only six great Councils of the Church that satisfy these conditions. For a list of them see COUNCIL.
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Miller, William James. Entry for 'Ecumenical'. The American Church Dictionary and Cyclopedia. https://www.studylight.org/dictionaries/eng/acd/e/ecumenical.html. 1901.