The Greek noun kanon (κανών) originally meant a straight rod or measuring rule — the instrument that determines the proper line or boundary. By extension it became a standard, norm, or rule by which things are measured. It appears four times in the New Testament, used by Paul to describe the scope of his apostolic mission and the standard by which believers should live.
Paul's use of kanon in 2 Corinthians 10:13–16 is fascinating: he describes the kanon — the measured field of ministry — that God has assigned him, warning against boasting beyond this boundary. In Galatians 6:16, 'those who follow this rule (kanon)' refers to the principle that neither circumcision nor uncircumcision matters, but new creation does. From these NT usages, the concept grew into the 'canon of Scripture' — the defined, ruled list of books recognized as authoritative. The kanon of Scripture is thus the 'measuring line' by which all other teaching is assessed.