A codified collection of the laws of the Catholic Church. The first Code, called Codex Iuris Canonici was issued in the early 20th Century. Before that, there were collections of Canon Law rules that were simply compilations of various Church decrees issued over many centuries.
Once the Code came out, the job of canon lawyers was simplified as all they needed was one book that contained all the laws of the Church. Naturally, there were additional decrees issued after the Code but these were much easier to keep track of.
The Code was completely revised and reissued, again under the name Codex Iuris Canonici about 20 years ago under the authority of Pope John Paul II.