Upvote:1
Ecumenical councils derive their authority from the pope who ratifies the councils' documents, so your question amounts to asking about papal infallibility or how a pope defines a dogma.
cf. also: "Creed, Encyclical, Decretal, Canon, Bull, etc - What's the difference?", "The Church's Theological Notes or Qualifications"
Also, St. Robert BellarmineβBook II, ch. XII of On Councils: Their Nature and Authority (from his De Controversiis)βcompares the authority of Holy Scriptures to that of councils:
Catholics do not subject the Sacred Scripture to Councils, but places it before them; nor is there any controversy on this point. But if some Catholics sometimes say scripture depends upon the Church, or a Council, they do not understand this in regard to its authority, or according to what it is, but in regard to the explanation and in regard to us.
(from this answer)