canon

[Collins Essential] 

1. (Christianity) a Church decree regulating morals or religious practices
   
2. a general rule or standard: the Marx-Engels canon
   

[Thesaurus/WordNet 3.0]

a rule (or especially body of rules or principles) generally established as valid and fundamental in a field or art or philosophy.

• "the neoclassical canon"; 
• "canons of polite society" 
• (Related words) prescript, rule -- prescribed guide for conduct or action

[Am-Heritage]

  3(a). An established principle: the canons of polite society.

  3(b). A basis for judgment; a standard or criterion.