Canonical

[Sarah]

The real deal; a piece that is widely recognized as a genuine member of the body of work (oeuvre) of a given artist/writer/composer; a standard by which all others are compared.

[Zipporah Schumaker]

The [standard form] of a mathmematical [equation] that can be written in many ways, making it easier to [compare] to other forms of the same equation.

[Sommer]

According to or ordered by canon law.

[Agnes]

The formal robes of a priest.

[Dorothey Allen]

Present in a canon, religious or otherwise.