The shaving of the head on acceptance into a religious community, in imitation of Christ's crown of thorns; its style however was a source of contention between branches of the early Christian Church. The Irish tonsure up to the 8c was different - the front of the scalp was shaved. The adoption of the Roman style was a sign of the final dominance of Roman over the Irish or Celtic Church. [< Lat. tondeo = to cut or shear]
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.