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 serve the 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.