"a rule or law," Old English canon "rule, law, or decree of the Church," from Old French canon or directly from Late Latin canon "Church law, a rule or doctrine enacted by ecclesiastical authority," in classical Latin, "measuring line, rule," from Greek kanon "any straight rod or bar; rule; standard of excellence," perhaps from kanna "reed" (see cane (n.)).
The Latin word was taken in ecclesiastical use for "decree of the Church." General sense of "a rule or principle" is from late 14c.; that of "standard of judging" is from c. 1600. From c. 1400 as "the Scriptures, the books of the Bible accepted by the Christian church," also extended to secular books of admitted excellence or supremacy. Meaning "catalogue of acknowledged saints" is from 1727. Musical sense "a kind of fugal composition" is from 1590s. Related: Canonicity.
The secular canon, with the word meaning a catalog of approved authors, does not actually begin until the middle of the eighteenth century .... [Harold Bloom, "The Western Canon," 1994].