A bhikkhu (Pali, Sanskrit: bhikṣu) is an ordained monastic (“monk”) in Buddhism. A female monastic (“nun”) is called a bhikkhuni (Sanskrit bhikṣuṇī).
A person under the age of 20 cannot be ordained as a bhikkhu or bhikkhuni but can be ordained as a śrāmaṇera or śrāmaṇērī.