In canon law, a canon designates some law promulgated by a synod, an ecumenical council, or an individual bishop.
The word "canon" comes from the Greek kanon, which in its original usage denoted a straight rod that was later the instrument used by architects and artificers as a measuring stick for making straight lines. Kanon eventually came to mean a rule or norm, so that when the first ecumenical council—Nicaea I—was held in 325, kanon started to obtain the restricted juridical denotation of a law promulgated by a synod or ecumenical council, as well as that of an individual bishop.
Etymology
Greek kanon / , Arabic / قانون, Hebrew kaneh / קנה, "straight"; a rule, code, standard, or measure; the root meaning in all these languages is "reed" (cf. the Romance-language ancestors of the English word "cane"). A kanon was the instrument used by architects and artificers for making straight lines.
Pre-Nicene usage
Some writers think that the Church preferred the word canon to law, as the latter had a harsh meaning for the faithful in the times of persecution.
The early Fathers use canon as equivalent to the rule of faith, or for some formula expressing a binding obligation on Christians.
Bickell declares that for the first three hundred years, canon is scarcely ever found for a separate and special decree of the Church; rather does it designate the rule of faith in general. He appeals to the fact that the plural form of the word is seldom used in the earliest Christian writers.
Nicene and Medieval usage
With the fourth century began the use of canon for a disciplinary decree, owing to its employment in this sense by the First Council of Nicea (325 A.D.).
The Cassinese editors of Ferraris say that in the first ages of the Church many disciplinary regulations were not required, and hence it was scarcely necessary to discriminate decrees into dogmatic and disciplinary, as the faithful classed both under the obligation to observe the general rule of faith.
Canon vs. decretum
According to the Catholic Encyclopedia, from the fourth century onward, canon signified almost universally a disciplinary decree of a council or of the Roman Pontiffs.
The word decretum during the same period, though signifying in general an authoritative statute or decision, began to be limited more and more to dogmatic matters, while canon when used in opposition to it was restricted to laws of discipline. That this usage, however, was not invariable is evident from Gratian's use of "Decretum" to signify his collection of canons and decrees.
Codifications
As ecclesiastical regulations began to multiply, it became necessary to gather them into codices, which generally received the title of "Collection of Canons". In these, civil laws are often added to the Church regulations. For such collections the Greeks used the term Nomocanones. The Latins have no special name for them, though the Capitularies, e. g. those of Charlemagne, are sometimes referred to as a somewhat parallel usage in the West.
Catholic Church
In 1917, the Catholic Church published the 1917 Code of Canon Law which applied to the Latin Church. In 1983, it published the a new Code of Canon Law for the Latin Church which replaced the 1917 Code of Canon Law.
During the reign of Pius XII, numerous canons for the Eastern Catholic Churches were published. In 1993, the Code of Canons of the Eastern Churches was published.
Eastern Orthodox Church
The Eastern Orthodox Church, principally through the work of 18th-century Athonite monastic scholar Nicodemus the Hagiorite, has compiled canons and commentaries upon them in a work known as the (, 'Rudder'), so named because it is meant to "steer" the Church in her discipline. The dogmatic determinations of the Councils are to be applied rigorously since they are considered to be essential for the Church's unity and the faithful preservation of the Gospel.
Catholic Church
In Catholic canon law, a canon is a certain rule or norm of conduct or belief prescribed by the Catholic Church.
Tridentine usage
From the First Council of Nicea up to the beginning of the Council of Trent, the regulations concerning discipline issued by assemblies of bishops received the name of canons.
With the Council of Trent in the sixteenth century began the departure from this ancient usage. This council used the word canon for short, dogmatic definitions with an anathema attached to them. On the other hand, it gave the name of decrees to its disciplinary regulations. The example set by Trent was followed by the First Council of the Vatican.