Source : Webster's Revised Unabridged Dictionary (1913)

Tonsure \Ton"sure\, n. [F., fr. L. tonsura a shearing, clipping,
   from tondere, tonsum, to shear, shave; cf. Gr. ? to gnaw;
   perhaps akin to Gr. ? to cut, and E. tome.]
   1. The act of clipping the hair, or of shaving the crown of
      the head; also, the state of being shorn.

   2. (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.

Source : WordNet®

tonsure
     n 1: the shaved crown of a monk's or priest's head
     2: shaving the crown of the head by priests or members of a
        monastic order
     v : shave the head of a newly inducted monk