A monk (, from Greek: μοναχός, monachos, "single, solitary" via Latin monachus) is a person who practices religious asceticism by living a monastic lifestyle, either alone or with any number of other monks. A monk may be a person who decides to dedicate their life to serving other people and serving God, or to be an ascetic who voluntarily chooses to leave mainstream society and live their 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. The word nun is typically used for female monastics.

Although the term monachos is of Christian origin, in the English language monk tends to be used loosely also for both male and female ascetics from other religious or philosophical backgrounds. However, being generic, it is not interchangeable with terms that denote particular kinds of monk, such as cenobite, hermit, anchorite, hesychast, or solitary.

Traditions of Christian monasticism exist in major Christian denominations, with religious orders being present in Catholicism, Lutheranism, Oriental Orthodoxy, Eastern Orthodoxy, Reformed Christianity (Calvinism), Anglicanism and Methodism. Indian religions, including Hinduism, Buddhism and Jainism, also have monastic traditions as well.