in various religions, a ceremony of initiation in which hair is clipped from the head as part of the ritual marking one's entrance into a new stage of religious development or activity.

Tonsure has been used in both the Roman Catholic and the Eastern Orthodox churches on occasions of solemn personal dedication to God. Until it was abolished by Pope Paul VI (effective in 1973), tonsure was the ceremony by which a man was initiated into the clerical state and became eligible for ordination to the priesthood. In the Eastern Orthodox church tonsure is part of the ordination of the lector (reader). In certain of the Eastern churches tonsure has also been a part of the ceremony admitting a man to the monastic life. The origins of the Christian use of this rite are not clear, but early Christian ascetics may have imitated the ancient religious practice among the Greeks and Semites that involved the cutting of some of the hair and offering it to a deity as a sign of dedication.

Three tonsures have been more or less in use in the Christian churches. The Roman, or St. Peter's, tonsure involved the shaving either of the whole head, with the exception of a fringe of hair supposed to symbolize the crown of thorns, or of a small round area at the crown of the head. In the Greek (Eastern, or St. Paul's) tonsure the whole head was shaved, but the more recent practice in the Eastern church has considered the tonsure adequate when the hair is merely shorn close. In the Celtic tonsure (tonsure of St. John, or, in contempt, of Simon Magus) all the hair in front of a line drawn over the top of the head from ear to ear was shaved.

In Buddhism tonsure is performed as a part of the ceremony of ordination as a novice (pravrajyā ceremony) and as a monk (upasaṃpadā ceremony). Thereafter, the monk keeps his head and face clean-shaven. In Myanmar (Burma) and Thailand most male children undergo the pravrajyā ceremony at about the age of eight or older and spend a few days or months in a monastery; the rite of tonsure is a principal part of the ceremony.

Jaina monks also cut their hair as a sign of renouncing the worldly life and entering the monkhood, traditionally, by plucking out the hairs one by one. Both Jaina and Buddhist customs are theoretically in imitation of the act performed by their teachers Mahāvīra and Gautama, who cut off their hair upon leaving their households to embark on the spiritual life.

In Hinduism the first tonsure undergone by a young boy (the ceremony of cūḍākaraṇa) is one of the saṃskāras, or personal sacraments, that mark the boy's transition from an infant to a child. It is usually performed when the boy is about two years old. The Hindu tonsure leaves a tuft of hair (the cūḍa) at the crown of the head. Tonsure formerly marked other rites of passage for the Hindu, such as the putting on of the sacred thread or the change of ritual status incurred by the death of the father (customs now largely observed only symbolically). Full tonsure is performed as part of the initiation rite into most Hindu ascetic orders.