Doctrine

Doctrine (from , meaning "teaching, instruction") 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 in a belief system. The etymological Greek analogue is "catechism".
Often the word "doctrine" specifically suggests a body of religious principles as promulgated by a church. "Doctrine" may also refer to a principle of law, in the common-law traditions, established through a history of past decisions.
Religious usage.
Examples of religious doctrines include:
Measure of religiosity.
According to sociologist Mervin Verbit, doctrine may be understood as one of the key components of religiosity. He divides doctrine into four categories: content, frequency (degree to which it may occupy the person's mind), intensity and centrality. Each of these may vary from one religion to the next, within that religious tradition.
In this sense, doctrine is similar to Charles Glock's "belief" dimension of religiosity.
Military usage.
The term also applies to the concept of an established procedure to a complex operation in warfare. The typical example is tactical doctrine in which a standard set of maneuvers, kinds of troops and weapons are employed as a default approach to a kind of attack.
Examples of military doctrines include:
Almost every military organization has its own doctrine, sometimes written, sometimes unwritten. Some military doctrines are transmitted through training programs. More recently, in modern peacekeeping operations, which involve both civilian and military operations, more comprehensive (not just military) doctrines are now emerging such as the 2008 United Nations peacekeeping operations' "Capstone Doctrine" which speaks to integrated civilian and military operations.
Political usage.
By definition, political doctrine is "[a] policy, position or principle advocated, taught or put into effect concerning the acquisition and exercise of the power to govern or administrate in society." The term political doctrine is sometimes wrongly identified with political ideology. However, doctrine lacks "the actional aspect" of ideology. It is mainly a theoretical discourse, which "refers to a coherent sum of assertions regarding what a particular topic should be" (Bernard Crick). Political doctrine is based on a rationally elaborated set of values, which may precede the formation of a political identity "per se". It is concerned with philosophical orientations on a "meta-theoretical" level.
Legal usage.
A legal doctrine is a body of interrelated rules (usually of common law and built over a long period of time) associated with a legal concept or principle. For example, the doctrine of frustration of purpose now has many tests and rules applicable with regards to each other and can be contained within a "bubble" of "frustration". In a court session a defendant may refer to the doctrine of justification.
It can be seen that a branch of law contains various doctrines, which in turn contain various "rules" or "tests". The test of "non-occurrence of crucial event" is part of the doctrine of "frustration" which is part of contract law. Doctrines can grow into a branch of law; restitution is now considered a branch of law separate to contract and tort.