If something’s canonical, it follows a principle or rule, usually in a religious or church-related situation. It is also used in mathematics, music and can refer to something reduced to its most basic form.
The word canonical is from the root canon, with both evolving from the Latin cononicus, or “according to rule,” a meaning applied to religion during the Middle Ages. However, the definition of “rules” also applies in other areas as well. In mathematics, the word is used to describe an equation reduced to its most basic form. In canonical music, a melody line is repeated at intervals throughout a piece. Perhaps the most familiar of these is Pachebel’s “Canon in D.”