(Latin: regula, rule)
In the proper, canonical sense of the word, religious men professed of the solemn vows, or the professed of simple vows in an order in which solemn vows are made, e.g., in the Society of Jesus.
In its less restricted and more general usage the word is applied to all religious men, both of orders and congregations; hence it is frequently used to designate priests who are also religious and thus bound by rule, in contradistinction to diocesan or secular clergy united by no rule.
When used in the phrase "Clerks Regular" the word refers to those religious, as the Theatines and the Jesuits, who are not monks in the proper use of the term, to distinguish them from the members of the older monastic orders, as the Benedictines.
New Catholic Dictionary