Canonical adjective [noun] (from the Greek κανών, kanón, “rule”) belonging to a particular canon of reference.
1 Conforms to the rule, to tradition. Synonyms: authorized, accepted, approved, recognized. For example: follow the canonical procedure.
2 In conformity with the canons of the Catholic Church: canon law.
3 Referred to a Sacred Text, included in the canon of Scriptures: canonical book.

Canon (priest) in Christian Church

From the Latin word canonicus, itself derived from the Greek κανονικός, kanonikós, “relating to a rule”, “regular”) is a member of certain bodies subject to an ecclesiastical rule (a clergyman).

Originally, a canon was a cleric living with others in a Monastery or a clergy house (or later in one of the houses within the precinct of or close to a cathedral and conducting his life according to the orders or rules of the church). This way of life (cenobitic) grew common in the early Christianity with Early Church Fathers and later on, around the XI century, some churches required clergy thus living together to adopt certain rules, as the ones proposed by Saint Augustine (or Saint Basil or Saint Benedict) renouncing to private wealth or making other Vovs to GOD. Those who embraced this change were known as Augustinians or Canons Regular, whilst those who did not were known as secular canons.