Doctrine is a legal principle that is widely adhered to. It is a rule or principle of the law established through the repeated application of legal precedents. Common law lawyers use this term to refer to an established method of resolving similar fact or legal issues as in "the doctrine of stare decisis".
Different branches of law contain various doctrines, which in turn contain various rules or tests. For example, the test of non-occurrence of crucial event is part of the doctrine of frustration which is part of contract law. Doctrines can even grow into a branch of law. For example, restitution is now considered a branch of law separate to contract and tort.