Doctrine, from Latin doctrina, (compare doctor), means "a body of teachings" or "instructions", taught principles or positions, as the body of teachings in a branch of knowledge or belief system. The Greek analogy is the etymology of catechism.
Often doctrine specifically connotes a corpus of religious dogma 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 matters of foreign policy, a doctrine is a body of axioms fundamental to the exercise of a nation's foreign policy. Hence, "doctrine,' in this sense, has come to suggest a broad consistency that holds true across a spectrum of acts and actions. Doctrines of this sort are almost always presented as the personal creations of one particular political leader, whom they are named after. Examples include the Monroe Doctrine, the Stimson Doctrine, the Truman Doctrine, the Eisenhower Doctrine, the Brezhnev Doctrine, and the less catchy Bush administration doctrine of military preeminence, and the Kirkpatrick doctrine.
In the US it has become something of a tradition for each President to have his own doctrine. See list of US Presidential Doctrines.
Sometimes 'doctrine' is an ambitious word for a mere slogan: the slogan 'Peace through strength' is termed a 'doctrine' at Wikipedia.
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 religious doctrines include:
See also the Roman Catholic Congregation for the Doctrine of the Faith.
Examples of military doctrines include:
See also Hallstein Doctrine.
Many more examples can be found at Wikipedia by searching doctrine * (doctrine and an asterisk).
The term indoctrination came to have awkward connotations during the 20th century, but it is necessary to retain it, in order to distinguish it from education. In education one is asked to stand as much as possible outside the body of accumulated knowledge and analyse it. In indoctrination on the other hand, one stands within the body of knowledge and absorbs its teachings. Compare theology and comparative religion for examples, of which many could be drawn.