I answer that, When a man enters the Church by Baptism, he is admitted to two things, viz. the body of the faithful and the participation of the sacraments: and this latter presupposes the former, since the faithful are united together in the participation of the sacraments. Consequently a person may be expelled from the Church in two ways. First, by being deprived merely of the participation of the sacraments, and this is the minor excommunication. Secondly, by being deprived of both, and this is the major excommunication, of which the above is the definition. Nor can there be a third, consisting in the privation of communion with the faithful, but not of the participation of the sacraments, for the reason already given, because, to wit, the faithful communicate together in the sacraments. Now communion with the faithful is twofold. One consists in spiritual things, such as their praying for one another, and meeting together for the reception of sacred things; while another consists in certain legitimate bodily actions. These different manners of communion are signified in the verse which declares that those who are excommunicate are deprived of---
"os, orare, vale, communio, mensa." "Os," i.e. we must not give them tokens of goodwill; "orare," i.e. we must not pray with them; "vale," we must not give them marks of respect; "communio," i.e. we must not communicate with them in the sacraments; "mensa," i.e. we must not take meals with them. Accordingly the above definition includes privation of the sacraments in the words "as to the fruit," and from partaking together with the faithful in spiritual things, in the words, "and the general prayers of the Church."
Another definition is given which expresses the privation of both kinds of acts, and is as follows: "Excommunication is the privation of all lawful communion with the faithful."