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Canon is the Greek word for rule, norm, standard or measure. It is used in several ways in church language:

1. The canon of Sacred Scripture is the list of books recognized by the church as inspired by the Holy Spirit.
2. Before the liturgical reforms of the Second Vatican Council, the single eucharistic prayer used universally in the Latin Mass was called the Roman Canon. Now that there are four eucharistic prayers in general use, they are usually referred to as Eucharistic Prayer I, II, III or IV, but they may also be called canons. The first of these is still called the Roman Canon because it is nearly identical to the original Roman Canon . 
3. Canon is another name for a law in the Code of Canon Law. (Adjective form is canonical .)

Canon Law is a code of ecclesiastical laws governing the Catholic Church. In the Latin or Western Church, the governing code is the 1983 Code of Canon Law. a revision of the 1917 Code of Canon Law. A separate but parallel Code of Canons of the Eastern Churches. issued in 1990, governs the Eastern Catholic churches. That document was the first comprehensive code of church law governing all Eastern Catholic churches.

Complementary Norms

When the revised Code of Canon Law was promulgated by Pope John Paul II in 1983, it contained eighty-four canons that call for or permit legislative action by episcopal conferences. Since that time the United States Conference of Catholic Bishops (designated the National Conference of Catholic Bishops prior to July 1, 2001) has taken action on twenty-nine canons, thereby establishing particular legislation for the dioceses of the United States.  These regional applications of Universal Canon Law that apply to a specific episcopal conference are referred to as Complementary Norms .