A monk (/mʌŋk/; from Greek: μοναχός, monachos, "single, solitary" via Latin monachus) is a man who is a member of a religious order and lives in a monastery. A monk usually lives his life in prayer and contemplation. The concept is ancient and can be seen in many religions and in philosophy across numerous cultures.

The Greek word for "monk" may be applied to men or women. In English, however, "monk" is applied mainly to men, while 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.^{[citation needed]} However, being generic, it is not interchangeable with terms that denote particular kinds of monk, such as cenobite, hermit, anchorite, or hesychast.

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, have monastic traditions as well.

In Theravada Buddhism, bhikkhu is the term for monk. Their disciplinary code is called the patimokkha, which is part of the larger Vinaya. They live lives of mendicancy, and go on a morning almsround (Pali: pindapata) every day. The local people give food for the monks to eat, though the monks are not permitted to positively ask for anything. The monks live in monasteries, and have an important function in traditional Asian society. Young boys can be ordained as samaneras. Both bhikkhus and samaneras eat only in the morning, and are not supposed to lead a luxurious life. Their rules forbid the use of money, although this rule is nowadays not kept by all monks. The monks are part of the Sangha, the third of the Triple Gem of Buddha, Dhamma, Sangha.

In Mahayana Buddhism, the term 'Sangha' strictly speaking refers to those who have achieved certain levels of understanding. They are therefore called 'community of the excellent ones' (Standard Tibetan: mchog kyi tshogs); however, these in turn need not be monks (i.e., hold such vows). Several Mahayana orders accept female practitioners as monks, instead of using the normal title of "nun", and they are considered equal to male ascetics in all respects.

The Bhikkhus are only allowed 4 items (other than their robes): a razor, a sewing needle, an alms bowl and a water strainer.^{[citation needed]}

In Vajrayana Buddhism, monkhood is part of the system of 'vows of individual liberation'; these vows are taken in order to develop one's own personal ethical discipline. The monks and nuns form the (ordinary) sangha. As for the Vajrayana vows of individual liberation, there are four steps: A lay person may take the 5 vows called 'approaching virtue' (in Tibetan 'genyen' < dge snyan>). The next step is to enter the monastic way of life (Tib. rabjung) which includes wearing monk's or nun's robes. After that, one can become a 'novice' (Pali samanera, Tib. getshül); the last and final step is to take all vows of the 'fully ordained monk' (gelong). This term 'gelong' (Tib. < dge long>, in the female form gelongma) is the translation of Skt. bikshu (for women bikshuni) which is the equivalent of the Pali term bhikkhuni; bhikkhu is the word used in Theravada Buddhism (Sri Lanka, Burma, Thailand).