Firmly-held opinions or well-tested ideas that describe a reproducable observation. A theory is often defined by a thesis statement that details the point of view or main ideas which organises structure according to the theorists perspective. Theories are the basis of science hypothesis.
A valid theory is one that has been confirmed by a number of experiments or investigations. Theories can be thought of as the connection between conjecture, reasoning and inference. Theoreticians can enjoy new discoveries by analysis of data alone.
One demonstration of the concept in science is the "theory" of evolution or tectonic shift. Although both are considered correct by the scientific community, they are not considered fact as they are not directly observable.