The word canonical is used to indicate a particular choice from of a number of possible conventions. This convention allows a mathematical object or class of objects to be uniquely identified or standardized.
For example, the right-hand rule for the cross product is a convention, which corresponds to the canonical vector
space orientation in 
.
Similarly, a canonical labeling orders or renames
the vertices of a graph so that isomorphic
graphs yield the same canonical form. Graph
isomorphism testing is thereby reduced to comparing the resulting canonically
labeled graphs.
