A bhikkhu (Pali, Sanskrit: bhikṣu) is an ordained monastic (“monk”) in Buddhism. A female monastic (nun) is called a bhikkhuni (Sanskrit bhikṣuṇī).
The lives of all Buddhist monastics governed by a set of rules called the prātimokṣa or pātimokkha. Their lifestyle is shaped to support their spiritual practice: to live a simple and meditative life and attain nirvana.
A person under the age of 20 cannot be ordained as a bhikkhu or bhikkhuni but can be ordained as a śrāmaṇera or śrāmaṇērī.