Q: What is a Canon?
A: “Canon” has a variety of meanings in the Episcopal Church, with one being especially salient during General Convention. Canon law is the list of rules by which the church is guided and governed, and national canons are changed by a meeting of the whole church at General Convention. This every-three-years meeting is happening right now in Louisville, Kentucky, with a full agenda. This is the only time when national canon law can be amended. These ecclesiastical laws set forth the rules and procedures by which the church and its members are to live. Of the 356 resolutions being considered during this General Convention, 121 of them pertain to canons. (paraphrasing from source below)
Additional meanings, all from A Dictionary for Episcopalians by John N. Wall.
Canon, cathedral: One who is on the staff of a cathedral and on its canonical roster, carrying out the work of the bishop in the diocese and the pastoral work of the cathedral itself. Cathedral canons can be both ordained and lay persons.
Canon of scripture: List of the Hebrew and Christian scriptures that are regarded as authoritative for the church. This list developed over time through a process of composition, circulation, revision, collection, and canonical recognition. Today the Old Testament canon exists in two forms: the Hebrew Bible is followed by Jews, Protestants, and some Orthodox churches, while the Greek Bible, or Septuagint, which includes the Apocrypha, is followed by Roman Catholics and some Orthodox churches. Bibles intended for public use in the Episcopal Church contain the canon of the Hebrew Bible, the Apocryphal books in a special section, and the canon of the New Testament. The canon of the New Testament was fixed in its present form by the end of the fourth century C.E.
Canon of the eucharist: Prayer over the elements, beginning with the Sursum corda, “Lift up your hearts.” It includes a seasonal or occasional preface; anamnesis, or remembering the story of salvation; the words of institution; epiclesis, or invocation of the Holy Spirit on the gifts; and a proclamation of Jesus’ lordship and his place in the trinity. Also known as the Great Thanksgiving.
Between the source above and the online version of An Episcopal Dictionary of the Church, A User Friendly Reference for Episcopalians, 3 related additional meanings are listed: Canon 9 ministers, canonical hours and canonical residence.