Doctrine (from Latin: doctrina, 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.

Examples of religious doctrines include:

Roman Catholic and Orthodox doctrine generally comes from the writings of the Church Fathers, which has been clarified in various Ecumenical councils. Short versions can be found in brief statements of Christian doctrine, in prayer books. Longer versions take the form of catechisms. Protestants generally reject Christian tradition and instead derive their doctrine solely from the Bible.

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.

The term also applies to the concept of an established procedure to execute an 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: