Tonsure
2 of the 7 encyclopedias on this shelf carry an entry for Tonsure. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
in ecclesiology and Church history, the shaving of the crown in a circle, which is a distinguishing mark of clerks in the Roman Church. Most of the mendicant and cloistered orders allow only a narrow strip of hair to grow round the head, all above and below being shaved; the tonsure of secular clerics is small. The tonsure is a necessary preliminary to entering the clerical state, whether secular or religious; in the former case it is conferred by the bishop of the diocese, in the latter by the head of the religious house, if a mitered abbot. It invests the receiver with all the privileges of a cleric, and furnishes a means to distinguish the higher from the lower clergy, as the extent of the tonsure increases with the rank till the priesthood is reached. Also, the act of admission to the clerical state. At first it was never given without some minor order being conferred at the same time, but this practice ceased in the 7th century.
Aiton's Encyclopedia (1910)
ton'shur, a shaven place on the head of a priest. The Roman Catholic tonsure or the tonsure of St. Peter consists of a bare circle on the top of the head. The first removal of the hair is performed by a bishop or cardinal and is one of the solemn rites of consecration to the priesthood. The candidate is pledged to shave the top of his head once a month. The higher the rank of a Catholic the wider may be his tonsure. The tonsure of the Greek Church , or the tonsure of St. Paul, consists in shaving, or at least clipping closely, the entire head. A third tonsure, the Celtic, or tonsure of St. John, consists in the removal of the hair in front of a line drawn over the top of the head from ear to ear.