The terminology used to qualify the modalities of excommunication may vary depending on the author.

The 1913 Catholic Encyclopedia distinguishes excommunication from the refusal of ecclesiastical communion, in which one bishop refuses to worship in common with another.

Anathema is a sort of aggravated excommunication, from which, however, it does not differ essentially, but simply in the matter of special solemnities and outward display.
Excommunication is either a jure (by law) or ab homine (by judicial act of man, i.e. by a judge). The first is provided by the law itself, which declares that whosoever shall have been guilty of a definite crime will incur the penalty of excommunication. The second is inflicted by an ecclesiastical prelate, either when he issues a serious order under pain of excommunication or imposes this penalty by judicial sentence and after a criminal trial.
Excommunication is either latæ sententiæ or ferendæ sententiæ.

Latae sententiae excommunication is incurred as soon as the offence is committed and by reason of the offence itself (eo ipso) without intervention of any ecclesiastical judge; it is recognized in the terms used by the legislator, for instance: "the culprit will be excommunicated at once, by the fact itself [statim, ipso facto]".

Ferendae sententiae excommunication is considered by the law as a penalty and is inflicted on the culprit only by a judicial sentence; in other words, the delinquent is rather threatened than visited with the penalty, and incurs it only when the judge has summoned him before his tribunal, declared him guilty, and punished him according to the terms of the law. It is recognized when the law contains these or similar words: "under pain of excommunication"; "the culprit will be excommunicated".
Excommunication ferendæ sententiæ can be public only, as it must be the object of a declaratory sentence pronounced by a judge; but excommunication latæ sententiæ may be either public or occult.
• An excommunication is public through the publicity of the law when it is imposed and published by ecclesiastical authority; it is public through notoriety of fact when the offence that has incurred it is known to the majority in the locality, as in the case of those who have publicly done violence to clerics, or of the purchasers of church property. This excommunication is valid in the forum externum and consequently in the forum internum.
• Excommunication is occult when the offence entailing it is known to no one or almost no one. This excommunication is valid in the forum internum only.
The practical difference of validities in the forums is very important:
• He who has incurred occult excommunication should treat himself as excommunicated and be absolved as soon as possible, submitting to whatever conditions will be imposed upon him, but this only in the tribunal of conscience; he is not obliged to denounce himself to a judge nor to abstain from external acts connected with the exercise of jurisdiction, and he may ask absolution without making himself known either in confession or to the Sacred Penitentiaria. …

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The Catholic Church claims that the penalty of excommunication is biblical and that both Paul of Tarsus and John the Apostle make reference to the practice of cutting people off from the community, in order to hasten their repentance. The Catholic Encyclopedia states that from the earliest days of Christianity, excommunication was the chief (if not the only) ecclesiastical penalty for laymen; for guilty clerics the first punishment was deposition from their office, i.e. reduction to the ranks of the laity. The Catholic Encyclopedia adds that during the first centuries of Christianity, excommunication was not regarded as a simple external measure, but also as one which touched the soul and the conscience. It was not merely the severing of the outward bond which holds individual to their place in the Church; it severed also the internal bond, and the sentence pronounced on earth was understood to be ratified in heaven.

During the Middle Ages, excommunication was analogous to the secular imperial ban or "outlawry" under common law. The individual was separated to some degree from the communion of the faithful. Formal acts of public excommunication were sometimes accompanied by a ceremony wherein a bell was tolled (as for the dead), the Book of the Gospels was closed, and a candle snuffed out—hence the idiom "to condemn with bell, book, and candle."

Those under excommunication were to be shunned. Pope Gregory VII was the first to mitigate the proscription against communicating with an excommunicated person. At a council in Rome in 1079, he made exceptions for members of the immediate family, servants, and occasions of necessity or utility. In the mid-12th century, Pope Eugene III held a synod in order to deal with the large number of heretical groups. Mass excommunication was used as a convenient tool to squelch heretics who belonged to groups which professed beliefs radically different than those taught by the Catholic Church.

William the Conqueror separated ecclesiastical cases from the Hundred courts, but allowed the bishops to seek assistance from the secular authorities. Excommunications were intended to be remedial and compel the offender to return to the fold. The practice in Normandy provided that if an obdurate excommunicate remained so for a year and a day, his goods were subject to confiscation at the duke's pleasure. Later, bishops were authorized to submit a writ to have the individual imprisoned. On the other hand, the bishops held temporalities which the king could seize if the bishop refused to absolve an imprisoned excommunicate. The authority of a bishop to excommunicate someone was restricted to those persons who resided in his see. This often gave rise to jurisdictional disput…

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In the Latin Church, canon law describes two forms of excommunication. The first is ferendae sententiae. This is where the person excommunicated is subject to a canonical process or trial, and if found guilty of misdemeanours meriting excommunication is duly sentenced. Once the sentence is published, that person is barred from active participation as a member of the Catholic Church. But this is a rare event.

The more common excommunication is that termed latae sententiae, or what sometimes called often "automatic excommunication", where someone, in committing a certain act, incurs the penalty without any canonical process having to take place. If the law or precept expressly establishes it, however, a penalty is latae sententiae, so that it is incurred ipso facto when the delict is committed (can. 1314).
A person may be ferendae sententiae (i.e., upon judicial review) excommunicated if the person
199 tries to celebrate the Mass without being a priest (incurs, for Latin Catholics, also a latae sententiae interdict for laymen and suspension for clerics, can. 1378 § 2 no. 1, 1983 CIC; can. 1443 CCEO);
299 hears a Confession or tries to absolve without being able to absolve (for Latin Catholics; this does not, of course, include hindrances on the penitent's side for the mere hearing of the Confessions, and hidden hindrances on the penitent's side for absolutions; can. 1378 § 2 no. 1, 1983 CIC; incurs also a latae sententiae interdict for laymen and suspension for clerics);
399 breaks the Seal of the Confessional indirectly or as someone not the Confessor, e.g. an interpreter or one who overheard something that was said (for Latin Catholics, can. 1388 § 2 1983 CIC);
499 who breaks a penal law allowing excommunication that was enacted on local level, which the local authority, however, may only do with great caution and for grave offences (for Latin Catholics, can. 1318, 1983 CIC);
599 omits stubbornly, as an Eastern Catholic priest, the commemoration of the hierarch in the Divine Liturgy and Divine Praises (not mandatorily, can. 1438 CCEO);
699 commits physical violence against a patriarch or a metropolitan, as an Eastern Catholic (can. 1445 § 1 CCEO);
799 incites sedition against any hierarch, especially a patriarch or the Pope, as an Eastern Catholic (can. 1447 § 1, 1983 CIC, not mandatorily);
899 commits murder, as an Eastern Catholic (can. 1450 § 1 CCEO);
999 kidnaps, wounds seriously, mutilates or tortures (physically or mentally) a person, as an Eastern Catholic (can. 1451 CCEO, not mandatorily);
1099 falsely accuses someone of a [canonical] offense, as an Eastern Catholic (can. 1454 CCEO, not mandatorily);
1199 tries to use the influence of secular authority to gain admission to Holy Orders or any function in the Church, as an Eastern Catholic (can. 1460 CCEO, not mandatorily);

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