Canon is a Latin word meaning “rule.” A canon is an established rule, or body of rules, for guidance.
In Biblical use it refers to the rule for which books of the Bible are canonical, that is, accepted for inclusion in the Bible. The two primary Scripture canons are the Palestinian Canon and the Alexandrian Canon.
In ecclesiastical use it describes a short definition of some dogmatic truth with an attached anathema, usually made by general councils.
In the Holy Sacrifice of the Mass, the Roman Canon is the First Eucharistic Prayer.
In religious life, certain orders of men with specific duties attached to a particular church, shrine, or ecclesiastical responsibility are called canons, that is, keepers of a rule.
In music, a composition that repeats the same melody by one or more voices in turn to produce harmony.
A list or catalog of canonized saints
The rule of a religious order. Some rules are called canons.
In art and architecture, an established rule in Church directives on ecclesiastical matters.
A priest or deacon attached to a cathedral, basilica, or other major church with specific duties such as the choral recitation of the Divine Office.
Photo above is Old St. Peter’s, ca. 326-1505,