What is Canon?

Legal Definition
For the legal system of canons, see Canon law and Canon law (Catholic Church).

In Catholic canon law, a canon is a certain rule or norm of conduct or belief prescribed by the Catholic Church. 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 ecumentical 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.
-- Wikipedia
Legal Definition
Eccl. law. This word is taken from the Greek, and signifies a rule or law. In ecelesiastical law, it is also used to designate an order of religious persons. Francis Duaren says, the reason why the ecclesiastics called the rules they established canons or rules, (canones id est regulas) and not laws, was modesty. They did not dare to call them (leges) laws, lest they should seem to arrogate to themselves the authority of princes and magistrates. De Sacris Ecclesiae Ministeriis, p. 2, in pref. See Law, Canon.
-- Bouviers Law Dictionary
Legal Definition
1. A law, rule or ordinance in general, and of the church in particular. An ecclesiastical law or statute.
See also
-- Black's Law Dictionary
Legal Definition
A law; a church officer who took revenue for conducting service.
-- Ballentine's Law Dictionary