What does "tonsure" mean?

Tonsure: (R. C. Ch.) (a) The first ceremony used for devoting a person to the service of God and the church; the first degree of the clericate, given by a bishop, abbot, or cardinal priest, consisting in cutting off the hair from a circular space at the back of the head, with prayers and benedictions; hence, entrance or admission into minor orders. (b) The shaven corona, or crown, which priests wear as a mark of their order and of their rank.