What is Doctrine?

Legal Definition
Doctrine (from Latin: doctrina) is a codification of beliefs or a body of teachings or instructions, taught principles or positions, as the essence of teachings in a given branch of knowledge or belief system. The Greek analogue is the etymology of catechism.

Often doctrine specifically suggests a body of religious principles as it is promulgated by a church, but not necessarily; doctrine is also used to refer to a principle of law, in the common law traditions, established through a history of past decisions, such as the doctrine of self-defense, or the principle of fair use, or the more narrowly applicable first-sale doctrine. In some organizations, doctrine is simply defined as "that which is taught", in other words the basis for institutional teaching of its personnel internal ways of doing business.
-- Wikipedia
Legal Definition
A single important rule or a set of rules that is widely followed in a field of law. In general, doctrines are simply rules or principles with such a long history in the law that lawyers and scholars have given them the more prestigious label of "doctrine."

More specifically, calling something a doctrine usually means at least one of two things: that it is very important to some field of law, or that it provides a comprehensive way to resolve a certain type of legal dispute.
Legal Definition
A rule, principle, theory or tenet of the law; as, the doctrine of merger, the doctrine of relation, etc.
-- Black's Law Dictionary