A bhikṣu is one who lives on alms got by begging. However, all beggars are not bhikṣus. The word is used more in a technical sense and applies to religious mendicants.
Renouncing the world and the worldly life in order to dedicate oneself to spiritual pursuits has been an ancient practice in India. Such persons generally moved about from place to place, sustaining themselves by begging.
Such a life of renunciation could be adopted from any stage of life—brahmacarya or the stage of a Vedic student, gārhasthya or the stage of a householder and vānaprastha or the stage of a forest dweller.
A bhikṣu represented the order of saṁnyāsa. He had to live a life of strict self-control and strive his utmost for mokṣa or liberation.
A Bhikṣusūtra attributed to one Karmanda has been mentioned in some dharmaśāstra works, but has not been traced. Another work of the same name attributed to Pārāśarya (Vyāsa) is perhaps the extant Brahmasūtras or an earlier version of it.
The word ‘bhikṣu’ in course of time, came to represent the Buddhist and the Jain mendicants, rather than the Hindu saṁnyāsins.