Summa summae S. Thomae, Tomus III

[By the name of council the same thing is signified in the Church as by the name of assemblies in a kingdom. For just as, when a controversy arises concerning things pertaining to the good of the commonwealth, the princes of the kingdom or magistrates gather together, or deputies elected by the people, that they may determine what needs to be done; so, when some controversy arisen in the Church concerning things pertaining to religion must be settled, the prelates of the particular churches come together, and, their opinions being compared at once, they determine what seems good. So we see it done by the Apostles, Acts, xv, where Christian councils began to be celebrated, and thereafter in each age.

The Latin name of council is found everywhere in the Old and New Testament for the assemblies of the Jews, but the Greek name, that is σύνοδος, is not found in the Scriptures; for where we read in Latin concilium, in the Greek text there is everywhere συνέδριον or συναγωγή. But the name synod appears first in the canons of the Apostles, canon 36, where bishops are commanded to hold councils twice each year; then it is read in Eusebius, book V of his History, chap. 23, and thereafter in all authors.

A council can be defined: « A gathering of ecclesiastical persons, especially bishops, convoked by legitimate authority, to determine what pertains to the religion and morals of Christians. »

We have the use of councils from the Apostles, as was said, and it is probable that their institution is divine, both from what is said, Matt., XVIII, 20: Where two or three are gathered together in my name, there am I in the midst of them; and because it is likely that the Apostles celebrated the first council either from the precept of Christ or from a special motion of the Holy Spirit, which was the norm of those that followed, and in which it was licit for them to say, Acts, xv, 28: It has seemed good to the Holy Spirit and to us.

Now there are four kinds of councils, namely general, national, provincial, and diocesan. General are called those to which the bishops of the whole world are convoked and can and ought to be present, unless legitimately impeded, and over which no one rightly presides except the Supreme Pontiff, or another in his name. Hence they are also called ecumenical, that is, councils of the whole earth. In national councils the archbishops and bishops of one kingdom or nation come together, over whom some patriarch or primate presides. Such were many Roman, Toledan, and African councils, concerning which it must be noted that, although they are distinguished from general and provincial ones, they are sometimes nevertheless called by the names of either, respectively with regard to the nation or kingdom. Provincial are properly called those in which only the bishops of one ecclesiastical province come together, over whom the archbishop or metropolitan presides. Diocesan are those in which only the priests of one bishopric come together, and over these presides the bishop, who alone possesses jurisdiction. Ordinarily gatherings of this kind are not adorned with the name of council, but are called synods, and they differ from councils in this, that the priests do not sit as judges, and have not a decisive voice but only a consultative one.]