A religious (using the word as a noun) is, in the terminology of many Western Christian denominations, such as the Catholic Church, Lutheran Churches, and Anglican Communion, what in common language one would call a "monk" or "nun", as opposed to an ordained "priest". A religious may also be a priest if he has undergone ordination, but in general he is not.
More precisely, a religious is a member of a religious order or religious institute, someone who belongs to "a society in which members...pronounce public vows...and lead a life of brothers or sisters in common".
Some classes of religious have also been referred to, though less commonly now than in the past, as regulars, because of living in accordance with a religious rule (regula in Latin) such as the Rule of Saint Benedict.
Religious are members of religious institutes, societies in which the members take public vows and live a fraternal life in common. Thus monks such as Benedictines and Carthusians, nuns such as Carmelites and Poor Clares, and friars such as Dominicans and Franciscans are called religious.
Those living other recognized forms of consecrated life are not classified as religious. A member of a secular institute is thus not a religious. Nor is a consecrated hermit, a consecrated virgin, or a person who follows some other form whose approval is reserved to the Holy See.
If a religious has been ordained as a deacon, a priest or a bishop, he also belongs to the clergy and so is a member of what is called the "religious clergy" or the "regular clergy". Clergy who are not members of a religious institute are known as secular clergy. They generally serve a geographically defined diocese or a diocese-like jurisdiction such as an apostolic vicariate or personal ordinariate, and so are also referred to as diocesan clergy.
A religious who has not been ordained is a member of the laity (a lay person), not of the clergy. However, once any non-ordained religious professes vows, especially final vows, they must be formally dispensed from those vows, which is a lengthy and formal process, with set procedures, that involves their local superior, the local Bishop or other Ordinary, the head of the Order, and the Vatican's Congregation for Religious. If they are ordained, they must also be formally suspended from and then relieved of their duties, and then laicized (formally removed from the clerical state), which is a related but separate matter. Both laicization and dispensation of vows are only done for very serious reasons, except for perhaps when one seeks to get married once it is done. The process is even more complex if they are accused of a secular or ecclesiastical offense or crime (some procedures can be expedited in certain criminal cases involving sex abuse). The state of a non-ordained religious, therefore, is not precisely the same as a lay unmarried person who is not a religious.
While the state of consecrated life is neither clerical or lay, institutes themselves are classified as one or the other. A clerical institute is one that "by reason of the purpose or design intended by the founder or by virtue of legitimate tradition, is under the direction of clerics, assumes the exercise of sacred orders, and is recognized as such by the authority of the Church". In clerical institutes, such as the Dominican Order or the Jesuits, most of the members are clerics. In only a few cases do lay institutes have some clergy among their members.
The 1983 Code of Canon Law devotes to religious 103 canons arranged in eight chapters:
In the Lutheran Churches, religious are defined as those who make religious vows before their bishop to live consecrated life, especially in a religious order. An ordained priest who is not a part of a Lutheran religious order is considered 'secular', rather than 'religious'.
In the Anglican Communion, the religious are those who have taken "vows of poverty, chastity, and obedience, usually in community".
Individuals called to a cloistered life are referred to as monks (men) and nuns (women), whereas men and women who are members of an order, but not living in cloister, are usually referred to by the term "religious," or "religious brothers" or "sisters. Examples of contemplative orders within the Roman Catholic tradition include, but are not limited to, Augustinian, Benedictine, Carthusian, Carmelite, and Cistercian. Among active orders in that same tradition are the Franciscans, Dominicans, and Jesuits. In addition the single form of monasticism in the Orthodox tradition, the Protestant tradition includes, but is not limited to, the following religious orders: the Order of St. Luke (Methodist Church); the Order of Lutheran Franciscans and the Congregation of the Servants of Christ (Lutheran); the Order of Julian of Norwich (Episcopal Church USA); the Order of St. Luke the Physician (Ecumenical); and the Knights of Prayer Monastic (Evangelical, Ecumenical).
Being a priest is a separate thing from being a monk, though monks can be priests (and so can Anglican and Lutheran nuns). But, basically, monks, whether priests or not, are 'religious' and distinct from those, priests or laypeople, who are 'secular'.