What is Canon Law?

Legal Definition
Canon law is the body of laws and regulations made by ecclesiastical authority (Church leadership), for the government of a Christian organization or church and its members. It is the internal ecclesiastical law, or operational policy, governing the Catholic Church (both the Latin Church and the Eastern Catholic Churches), the Eastern Orthodox and Oriental Orthodox churches, and the individual national churches within the Anglican Communion. The way that such church law is legislated, interpreted and at times adjudicated varies widely among these three bodies of churches. In all three traditions, a canon was originally a rule adopted by a church council; these canons formed the foundation of canon law.
-- Wikipedia
Legal Definition
The canon law is a body of Roman ecclesiastical law, relative to such matters as that church either has or pretends to have the proper jurisdiction over:

2. This is compiled from the opinions of the ancient Latin fathers, the decrees of general councils, and the decretal epistles and bulls of the holy see. All which lay in the same confusion and disorder as the Roman civil law, till about the year 1151, when one Gratian, an Italian monk, animated by the discovery of Justinian's Pandects, reduced the ecclesiastical constitutions also into some method, in three books, which he entitled Concordia discordantium canonum, but which are generally known by the name of Decretum Gratiani. These reached as low as the time of Pope Alexander III. The subsequent papal decrees to the pontificate of Gregory IX., were published in much the same method, under the auspices of that pope, about the year 1230, in five books, entiled Decretalia Gregorii noni. A sixth book was added by Boniface VIII., about the year 1298, which is called Sextus decretalium. The Clementine constitution or decrees of Clement V., were in like manner authenticated in 1317, by his successor, John XXII., who also published twenty constitutions of his own, called the Extravagantes Joannis, all of which in some manner answer to the novels of the civil law. To these have since been added some decrees of the later popes, in five books called Extravagantes communes. And all these together, Gratian's Decrees, Gregory's Decretals, the Sixth Decretals, the Clementine Constitutions, and the Extravagants of John and his successors, form the Corpus juris canonici, or body of the Roman canon law. 1 Bl. Com. 82; EncyclopÇdie, Droit Canonique, Droit Public Ecclesiastique; Dict. de Jurispr. Droit Canonique; Ersk. Pr. L. Scotl. B. 1, t. 1, s. 10. See, in general, Ayl. Par. Jur. Can. Ang.; Shelf. on M. & D. 19; Preface to Burn's Eccl. Law, by Thyrwhitt, 22; Hale's Hist. C. L. 26-29; Bell's Case of a Putative Marriage, 203; Dict. du Droit Canonique; Stair's Inst. b. 1, t. 1, 7.
-- Bouviers Law Dictionary
Legal Definition
A body of ecclesiastical jurisprudence which, in countries where the Roman Catholic church is established, is composed of maxims and rules drawn from patristic sources, oniinances and decrees of general councils, and the decretals and bulls of the popes. In England, according to Blackstone, there is a kind of national canon law, composed of legatine and provincial constitutions enacted in England pnor to the reformation, and adapted to the exigencies of the English church and kingdom. 1 Bl. Comm. 82. The canon law consists partly of certain rules taken out of the Scripture, partly of the writings of the ancient fathers of the church, partly of the ordinances of general and provincial councils, and partly of the decrees of the popes in former ages; and it is contained in two principal parts,
-- Black's Law Dictionary
Legal Definition
The law of the Roman church.
-- Ballentine's Law Dictionary