In a paper I am working on, I come across a term called  "graph canonisation "

According to wikipedia : 

>In graph theory, a branch of mathematics, graph canonization is the problem finding a canonical form of a given graph $G$. A canonical form is a labeled graph Canon$(G)$ that is isomorphic to G, such that every graph that is isomorphic to $G$ has the same canonical form as $G$.

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According to math-world Wolfram : 


>A canonical labeling, also called a canonical form, of a graph G is a graph $G^{'}$ which is isomorphic to G and which represents the whole isomorphism class of G (Piperno 2011). The complexity class of canonical labeling is not known

Could one elaborate on that ?

**Motivation** : I am working on graph isomorphism.