ब्रह्मचारिन्, brahmacārin

A brahmacharin is one who follows brahmacharya, one of the parts of yama that includes chastity.

An unmarried disciple practicing temperance, strictly observing certain vows and religious disciplines, including sadhana, devotional service and obedience to a spiritual mentor. 

Brahmacharins usually live in an ashram with their Guru. At the end of brahmacharya period (spiritual apprenticeship and obedience), the brahmacharin either becomes a married secular (grihastha), or takes full monastic vows, becoming a sannyasin. 

The same word is often used as a reference to an unmarried young man between the ages of 12 and 24 (or even up to marriage).