A monk (from Greek: μοναχός, monachos, "single, solitary") is a person who practices religious asceticism, living either alone or with any number of other monks. A monk may be a person who decided to dedicate his life to serving all other living beings, or to be an ascetic who voluntarily chooses to leave mainstream society and live his life in prayer and contemplation. The concept is ancient and can be seen in many religions and in philosophy.
In the Greek language the term can apply to women, but in modern English it is mainly in use for men. Meanwhile, nun is typically used for female monastics.
Although the term monachos ("monk") is of Christian origin, in the English language it tends to be used analogously or loosely also for both male and female ascetics from other religious or philosophical backgrounds.
The term monk is generic and in some religious or philosophical traditions it therefore may be considered interchangeable with other terms such as ascetic. However, being generic, it is not interchangeable with terms that denote particular kinds of monk, such as friar, cenobite, hermit, anchorite, hesychast, solitary.
In the Eastern Orthodoxy and Oriental Orthodoxy monasticism holds a very special and important place: "Angels are a light for monks, monks are a light for laymen" (St. John Klimakos). The Orthodox Church measures its health by the quality of its monks and nuns. Orthodox monastics separate themselves from the world in order to pray unceasingly for the world. They do not, in general, have as their primary purpose the running of social services, as is common in Western Christianity, but instead are concerned with attaining theosis, or union with God. However, care for the poor and needy has always been an obligation of monasticism, so Orthodox monasteries are not normally "cloistered" like some contemplative Western houses are, though the level of contact will vary from community to community. Orthodox hermits, on the other hand, have little or no contact with the outside world.