What is "plenary council"

A plenary council, in the Roman Catholic Church, is a term applied to various kinds of ecclesiastical synods, used when those summoned represent the whole number of bishops of some given territory. The word itself, derived from the Latin plenarium (complete or full), hence concilium plenarium, also concilium plenum.

The ecumenical councils or synods are called plenary councils by Augustine of Hippo, as they form a complete representation of the entire Church. Thus also, in ecclesiastical documents, provincial councils are denominated plenary, because all the bishops of a certain ecclesiastical province were represented. Later usage has restricted the term plenary to those councils which are presided over by a delegate of the Apostolic See, who has received special power for that purpose, and which are attended by all the metropolitans and bishops of some commonwealth, empire, or kingdom, or by their duly accredited representatives. Such plenary synods are frequently called national councils.