A religious (using the word as a noun) is, in the terminology of the Catholic Church, 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 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.
Ordination as deacon, priest or bishop does not make one a member of a religious institute and so does not make one a religious.