In the past, what distinguished religious orders from other institutes was the classification of the vows that the members took in religious profession as solemn vows, but in the course of the 20th century some religious institutes outside the category of orders obtained permission to make solemn vows, at least of poverty, thus blurring the distinction.
Solemn vows were originally considered indissoluble. As noted below, dispensations began to be granted in later times, but originally not even the Pope could dispense from them. If for a just cause a religious was expelled, the vow of chastity remained unchanged and so rendered invalid any attempt at marriage, the vow of obedience obliged in relation, generally, to the bishop rather than to the religious superior, and the vow of poverty was modified to meet the new situation but the expelled religious "could not, for example, will any goods to another; and goods which came to him reverted at his death to his institute or to the Holy See".
Weakening in 1917
The 1917 Code of Canon Law reserved the name "religious order" for institutes in which the vows were solemn, and used the term "religious congregation" or simply "congregation" for institutes with simple vows. The members of a religious order for men were called "regulars", those belonging to a religious congregation were simply "religious", a term that applied also to regulars. For women, those with simple vows were called "sisters", with the term "nun" reserved in canon law for those who belonged to an institute of solemn vows, even if in some localities they were allowed to take simple vows instead.
However, it abolished the distinction according to which solemn vows, unlike simple vows, were indissoluble. It recognized no totally indispensable religious vows and thereby abrogated for the Latin Church the special consecration that distinguished "orders" from "congregations", while keeping some juridical distinctions.
In practice, even before 1917 dispensations from solemn religious vows were being obtained by grant of the Pope himself, while departments of the Holy See and superiors specially delegated by it could dispense from simple religious vows.
The 1917 Code maintained a juridical distinction by declaring invalid any marriage attempted by solemnly professed religious or by those with simple vows to which the Holy See had attached the effect of invalidating marriage, while stating that no simple vow rendered a marriage invalid, except in the cases in which the Holy See directed otherwise. Thus members of "orders" were barred absolutely from marriage, and any marriage they attempted was invalid. Those who made simple vows were obliged not to marry, but if they did break their vow, the marriage was considered valid.
Another difference was that a professed religious of solemn vows lost the right to own property and the capacity to acquire temporal goods for himself or herself, but a professed religious of simple vows, while being prohibited by the vow of poverty from using and administering property, kept ownership and the right to acquire more, unless the constitutions of the religious institute explicitly stated the contrary.
After publication of the 1917 Code, many institutes with simple vows appealed to the Holy See for permission to make solemn vows. The Apostolic Constitution Sponsa Christi of 21 November 1950 made access to that permission easier for nuns (in the strict sense), though not for religious institutes dedicated to apostolic activity. Many of these latter institutes of women then petitioned for the solemn vow of poverty alone. Towards the end of the Second Vatican Council, superiors general of clerical institutes and abbots president of monastic congregations were authorized to permit, for a just cause, their subjects of simple vows who made a reasonable request to renounce their property except for what would be required for their sustenance if they were to depart. These changes resulted in a further blurring of the previously clear distinction between "orders" and "congregations", since institutes that were founded as "congregations" began to have some members who had all three solemn vows or had members that took a solemn vow of poverty and simple vows of chastity and obedience.
Further changes in 1983
The current Code of Canon Law, which came into force in 1983, maintains the distinction between solemn and simple vows, but no longer makes any distinction between their juridical effects, including the distinction between "orders" and "congregations". It has accordingly dropped the language of the 1917 code and uses the single term "religious institute" (which appears nowhere in the 1917 Code) to designate all such institutes of consecrated life alike.
Thus the Church no longer draws the historical distinction between religious "orders" and "congregations". It applies to all such institutes the single name "religious institute" and the same rules of canon law. While solemn vows once meant those taken in what was called a religious order, "today, in order to know when a vow is solemn it will be necessary to refer to the proper law of the institutes of consecrated life."
"Religious order" and "religious institute" tend indeed to be used now as synonyms, and canon lawyer Nicholas Cafardi, commenting on the fact that the canonical term is "religious institute", can write that "religious order" is a colloquialism.
Thomas Schoen 1903, OCist.
A religious order is characterized by an authority structure where a superior general has jurisdiction over the order's dependent communities. An exception is the Order of St Benedict which is not a religious order in this technical sense, because it has a system of "independent houses", meaning that each abbey is autonomous. However, the Constitutions governing the order's global "independent houses" and its distinct "congregations" (of which there are twenty) were approved by the pope. Likewise, according to rank and authority, the abbot primate's "position with regard to the other abbots [throughout the world] is to be understood rather from the analogy of a primate in a hierarchy than from that of the general of an order like the Dominicans and Jesuits."
The Canons Regular of Saint Augustine are in a situation similar to that of the Benedictines. They are organized in eight "congregations", each headed by an "abbot general", but also have an "Abbot Primate of the Confederated Canons Regular of Saint Augustine". And the Cistercians are in thirteen "congregations", each headed by an "abbot general" or an "abbot president", but do not use the title of "abbot primate".
List of institutes of consecrated life in the Annuario Pontificio
The Annuario Pontificio lists for both men and women the institutes of consecrated life and the like that are "of pontifical right" (those that the Holy See has erected or approved by formal decree). For the men, it gives what it now calls the Historical-Juridical List of Precedence. The arrangement in this list dates back many decades. It is found, for instance, in the 1964 edition of the Annuario Pontificio, pp. 807–870, where the heading is "States of Perfection (of pontifical right for men)". In the 1969 edition the heading has become "Religious and Secular Institutes of Pontifical Right for Men", a form it kept until 1975 inclusive. Since 1976, when work was already advanced on revising the Code of Canon Law, the list has been qualified as "historical-juridical" and still includes as "orders" the following institutes for men of the Latin Church, while not distinguishing between "orders" and "congregations" in the case of the Eastern Catholic Churches and Latin Church women.
Within that long list, a relatively small section is devoted to Latin-Rite "orders" for men:
The 2012 Annuario Pontificio, which devotes 19 pages to this information on Latin-Rite "orders" for men, gives 35 pages to Latin-Rite "congregations" for men, 7 to Eastern "orders, religious congregations and societies of apostolic life" for men, and 198 pages to more concise information on religious institutes for women.
Thomas Aquinas, Summa Theologica, II-II, q. 88, a.11
^ Paul M. Quay, "Renewal of Religious Orders, or Destruction?", in Commentarium pro Religiosis et Missionariis, vol. 65 (1984), pp. 77-86
1917 Code of Canon Law, canon 488
William Edward Addis, Thomas Arnold, A Catholic Dictionary Containing Some Account of the Doctrine, Discipline, Rites, Ceremonies, Councils and Religious Orders of the Catholic Church, Part Two, p. 858 (reprinted by Kessinger Publishing 2004)
1917 Code of Canon Law, canon 1073
1917 Code of Canon Law, canon 1058
1917 Code of Canon Law, canons 580-582
Yūji Sugawara, Religious Poverty: from Vatican Council II to the 1994 Synod of Bishops (Loyola Press 1997 ISBN 978-88-7652-698-5), pp. 127-128
Code of Canon Law, canon 1192 §2
IntraText concordance to the 1917 Code
Robert T. Kennedy, Study related to a pre-1983 book by John J. McGrath – Jurist, 1990, pp. 351-401
Code of Canon Law, canons 607-709
E. Caparros, M. Thériault, J. Thorne (editors), Code of Canon Law Annotated (Wilson & Lafleur, Montréal 1993 ISBN 2-89127-232-3), p. 745
Article published in Theological Exploration, vol. 2. no. 1 of Duquesne University and in Law Review of University of Toledo, vol 33
See "The Benedictine Order" in New Advent, Catholic Encyclopedia
Code of Canon Law, canon 589
Annuario Pontificio 2008 (Libreria Editrice Vaticana 2012 ISBN 978-88-209-8722-0), pp. 1411-1468