Canon law is the body of law by which a Christian church orders itself, from the Greek kanon, a rule or measuring line. It is oldest, largest and most developed in the Roman Catholic Church, where it constitutes a complete legal system with its own courts, procedure, judges, advocates, faculties and academic literature.
Its formation ran for a millennium. The canons of ecumenical and local councils from the fourth century onward, papal decretals, and patristic rulings were gathered into collections of increasing sophistication and decreasing contradiction. The decisive work is Gratian's Decretum of about 1140, whose full title, Concordia discordantium canonum, the harmony of discordant canons, states its method: to set conflicting authorities side by side and reconcile them by distinction. It effectively founded canon law as a science, taught at Bologna alongside the rediscovered Roman law, and the two together shaped the European legal profession. Later decretal collections were consolidated as the Corpus Iuris Canonici, which governed until the first Code of Canon Law in 1917. That code was replaced by the 1983 Code promulgated after the Second Vatican Council, with a separate Code of Canons of the Eastern Churches (1990) for the Eastern Catholic churches.
Its scope is wide: the sacraments and who may administer and receive them; marriage law, including the grounds on which a marriage may be declared null; the structure of dioceses, parishes and religious institutes; the appointment, transfer and removal of clergy; ecclesiastical property; and a penal book with its own offences and procedure. The 1983 Code opens its treatment of the people of God with the obligations and rights of all the faithful before turning to the clergy, a reordering that reflects the council's ecclesiology, and it ends with the maxim that the salvation of souls must always be the supreme law.
Other churches order themselves differently. Eastern Orthodoxy accords great authority to the ancient canons but has no single code, working from the conciliar canons, collections such as the Pedalion, and the statutes of each autocephalous church. The Anglican churches each have their own canons, English canon law retaining much of its medieval inheritance as modified at the Reformation. Most Protestant churches speak of church order, polity or discipline rather than canon law.
Two points are worth adding. Canon law is one of the two great learned legal traditions of medieval Europe, and its influence on the development of Western secular law, in procedure, evidence, the law of corporations, contract, and marriage, is substantial and well documented. And its handling of clerical sexual abuse has been the object of sustained and largely justified criticism: the penal canons were little used, the norms of secrecy impeded response, and the procedures were widely judged inadequate by the church's own later admission. The penal book was substantially rewritten in 2021.