Theory it's the set of fundamental principles of an art or a science. Theory is a synthesized opinion, is a general notion. from the greek theory which in the historical context meant to observe or examine. With its evolution, the term came to designate the set of ideas, the basis of a given theme, which seeks to convey a general notion of some aspects of reality.
Theory is also a hypothesis, a conjecture, an opinion formed in the face of a fact. A theory tries to explain something difficult to achieve.
At area of Mathematics, theory is any proposition which, in order to be admitted, needs proof.
One scientific theory it is the speculative part of a science, as opposed to the practice. It is a consistent system formed by observations, ideas and axioms or postulates, constituting as a whole a set that tries to explain certain phenomena.
are examples of scientific theories: Chaos Theory, Communication Theory, Knowledge Theory, Evolution Theory, Game Theory, Quantum Theory, Relativity Theory, among others.
In philosophy, teoria is the set of knowledge that present different degrees of systematization and credibility, and that propose to elucidate, interpret or explain a phenomenon or event that is offered to the activity practice.