Because this question came up in the meeting when I passed out the Church Councils overview... Here is the definition of an "Ecumenical Council" according to the Catholic Encyclopedia:
Ecumenical Councils are those to which the bishops, and others entitled to vote, are convoked from the whole world (oikoumene) under the presidency of the pope or his legates, and the decrees of which, having received papal confirmation, bind all Christians. A council, Ecumenical in its convocation, may fail to secure the approbation of the whole Church or of the pope, and thus not rank in authority with Ecumenical councils. Such was the case with the Robber Synod of 449 (Latrocinium Ephesinum), the Synod of Pisa in 1409, and in part with the Councils of Constance and Basle.
