A theory is the more impressive the greater the simplicity of its premises is, the more different kinds of things it relates, and the more extended its area of applicability. — Albert Einstein, Albert Einstein: Philosopher-Scientist
Theory is especially often contrasted to “practice” an Aristotelian concept which is used in a broad way to refer to any activity done for the sake of action, in contrast with theory, which does not need an aim which is an action. A classical example of the distinction between theoretical and practical uses the discipline of medicine: Medical theory and theorizing involves trying to understand the causes and nature of health and sickness, while the practical side of medicine is trying to make people healthy. These two things are related but can be independent, because it is possible to research health and sickness without curing specific patients, and it is possible to cure a patient without knowing how the cure worked. — Wikipedia
Metatheory is a theory whose subject matter is some theory. All fields of research share some meta-theory, regardless whether this is explicit or correct. In a more restricted and specific sense, in mathematics and mathematical logic, metatheory means a mathematical theory about another mathematical theory. — Wikipedia
Here are links to pages about closely related subjects.
Philosophy Metaphysics, Logic, Epistemology, Ethics, Aesthetics
System, Cycle, Structure, Growth, Complexity, Control, Disturbance, Entropy, Chaos
Process, Manage, Design, Prototype, Implement, Assess, Revise, Maintain