A theory is a metascientific elaboration, distinct from the results of observation but consistent with them. By means of it a series of independent data and facts can be related and interpreted in a unified explanation. A theory's validity depends on whether or not it can be verified, it is constantly tested against the facts; wherever it can no longer explain the latter, it shows its limitations and unsuitability. It must then be rethought. 
If a theory or point of view is treated only as doctrine to be validated, and not one to be challenged, it is not within the realm of science.
The term "theory" is often used outside of the realm of natural science, such as in music theory, number theory, game theory, or Marxist theory. In this usage, theory is used to refer to an area of study.