What is Chapter?

Legal Definition
Eccl. law. A congregation of clergymen. Such an assembly is termed capitulum, which signifies a little head it being a kind of head, not only to govern the diocese in the vacation of the bishopric, but also for other purposes. Co. Litt. 103.
-- Bouviers Law Dictionary
Legal Definition
In ecclesiastical law. A congregation of ecclesiastical persons in a cathedral church, consisting of canons or prebendaries, whereof the dean is the head, all subordinate to the bishop, to whom they act as assistants in matters relating to the church, for the better ordering and disposing the things thereof, and the confirmation of such leases of the temporally and offices relating to the bishopric, as the bishop shall make from time to time. And they are termed "capitulum," as a kind of head, instituted not only to assist the bishop in manner aforesaid, but also anciently to rule and govern the diocese in the time of vacation. Burn, Diet.
-- Black's Law Dictionary
Legal Definition
A bishop’s council.
-- Ballentine's Law Dictionary