Legal system of a church, such as the Catholic Church. So named because it is a collection of canons, or written (legal) rules.
Specifically, the Canon Law of the Catholic Church has developed over many centuries and contained so many canons that it was quite impossible for any one individual to know them. The profession of canonist developed - an expert who studied the various canons.
Somewhere around the 19th Century various canons were collected and published. In the early 20th Century, the Catholic Church published its first Code of Canon Law, in which all the various canons were made into a system. In the late 20th Century, the Code of Canon Law was revised, and a new one published, signed to law by Pope John Paul II.