Prior, derived from the Latin for "earlier, first", (or prioress for nuns) is an ecclesiastical title for a superior, usually lower in rank than an abbot or abbess. Its earlier generic usage referred to any monastic superior.
In the Rule of Saint Benedict, the term appears several times, referring to any superior, whether an abbot, provost, dean, etc. In other old monastic rules the term is used in the same generic sense.
With the Cluniac Reforms, the term prior received a specific meaning; it supplanted the provost or dean (praepositus), spoken of in the Rule of St. Benedict. The example of the Cluniac congregations was gradually followed by all Benedictine monasteries, as well as by the Camaldolese, Vallombrosians, Cistercians, Hirsau congregations, and other offshoots of the Benedictine Order.
Monastic congregations of hermit origin generally do not use the title of abbot for the head of any of their houses, in an effort to avoid the involvement with the world the office of an abbot would entail. As a result, it is not in use for the congregation as a whole. Among them, the equivalent term of prior general is the one used. This applies, e.g., for the Camaldolese and the Carthusians.
The term is also used by various mendicant orders, e.g., the Carmelites and the Dominicans. This applies both to the friars and the nuns of these orders.
In the Benedictine Order and its branches, in the Premonstratensian Order, and in the military orders there are three kinds of priors: the claustral prior, the conventual prior, and the obedientiary prior.