A bhikkhu (in Pali) or bhikshu (in Sanskrit) is a Buddhist monk; the female version is bhikkhuni (bhikshuni in Sanskrit), though some Buddhist traditions - the Theravadan and the Tibetan - do not recognize an existing order of bhikkhuni.
In Buddhist terms, the bhikkhu and bhikkhuni are conceived of as regular people - householders - who have left that life to pursue a path of heightened virtue, renouncing worldly concerns. The community of bhikkhu and bhikkhuni are known as the sangha, and they live according to the Vinaya, an exhaustive list of monastic rules which spans 6 printed volumes and defines every aspect of their way of life. The conjunction of the Dhamma or Dharma with the Vinaya forms the core of the Buddhism; Dhamma-vinaya, Pali for the doctrine and discipline, is the name the Buddha gave to the system of practices he founded.