A canon is a set of rules that applies to proportions in sculpture. The basic premise is to create an ideal human figure by determining what the proportions of one part of the human figure 'ideally' should be with relationship to the other parts of the body. The Greek sculpture Discophorus is a good example of this concept, as is Leonardo da Vinci's Vitruvian Man.
Another related concept of this striving for perfection is the golden section or golden mean.