Tonsure is the practice of some Christian churches of cutting the hair from the scalp of clerics. There were three forms of tonsure known in the seventh and eight centuries:
(1) The Oriental, which claimed the authority of St. Paul and consisted in shaving the whole head. This was observed by churches owing allegiance to Eastern Orthodoxy. Hence Theodore of Tarsus, who had acquired his learning in Byzantine Asia Minor and bore this tonsure, had to allow his hair grow for four months before he could be tonsured after the Roman fashion, and then ordained Archbishop of Canterbury by Pope Vitalian in 668.
(2) The Celtic, this consisted in shaving the whole front of the head from ear to ear, the hair being allowed to hang down behind. The Roman party in Britain attributed the origin of this tonsure to Simon Magnus, though some traced it back to the swineherd of Loegaire, the Irish king who opposed St. Patrick. The fact that it was common to all of the Celts, both insular and continental, is a sufficient refutation of the latter view. Some practicers of Celtic Christianity claimed for this, as for their Easter practices, the authority of St. John. It is entirely plausible that the Celts were merely observing an older practice which had become obsolete elsewhere.
(3) The Roman, this consisted in shaving only the top of the head, so as to allow the hair to grow in the form of a crown. This was the practice of the Catholic church until it was abolished in 1972. The origin of this practice is claimed to have originated with St. Peter.
It is needless to say that these claimed origins are unhistorical; the early history of the tonsure is lost in obscurity. This practice is not improbably connected with the Roman idea that long hair is the mark of a freeman, while the shaven head marks the slave.
Based on Charles Plummer's essay, "Excusrsus on the Pascal Controversy and Tonsure" (in his edition of Bede's Opera Historica, 1898).
Today in Eastern Orthodoxy, tonsure refers to the initiation rite when someone becomes a monk, nun, reader, or subdeacon; the rite is analogous to ordination, which is the initiation rite for deacons, priests and bishops. Tonsure usually includes cutting a small token amount of hair.