Upanayana (Sanskrit: उपनयन, meaning 'initiation') is a Hindu educational sacrament and one of the traditional saṃskāras, or rites of passage, in Hinduism. The ceremony marks the formal acceptance of a student by a preceptor, such as a guru or acharya, and signifies the individual's initiation into a school of learning.
In several traditions, the rite is regarded as a spiritual rebirth, with the initiate being described as a dvija, or twice born. It signifies the beginning of the acquisition of knowledge and the commencement of a disciplined life as a brahmāchārya. The ceremony is considered an important rite for Brāhmaṇa, Kṣatriya, and Vaiśya males, marking the assumption of their rights and responsibilities and their entry into adulthood. Typically, the ceremony is performed before the advent of adulthood.
The tradition is discussed extensively in ancient Sanskrit texts of Hinduism, and its observance varies regionally. In contemporary practice, the sacred thread, known as the yajñopavīta — and also referred to as Janeu, Jandhyam, Pūṇūl, Muñja, Janivara, or Yonya in different regions and languages — has become one of the most prominent identifiers of the Upanayana ceremony, though this was not always central to the rite historically.
Adapted from the English Wikipedia article on Upanayana.