The word "theory" derives from the Greek word "theoria", which means "to look at". According to Wikipedia, theory has a number of meanings depending on the context of discussion. In everyday life, people use the word "theory" to signify "speculation", "opinion" or "hypothesis". Therefore, "theories" are not necessarily facts. However, in the field of science, a theory is considered as a well established and verifiable model of interaction of a set of phenomena, capable of predicting future occurrences and, its outcome can not be challenged.
National Academy of Sciences describes the word theory in the following way: Some scientific explanations are so well established that no new evidence is likely to alter them. The explanation becomes a scientific theory. In everyday language a theory means a hunch or speculation. Not so in science. In science, the word theory refers to a comprehensive explanation of an important feature of nature that is supported by many facts gathered over time. Theories also allow scientists to make predictions about as yet unobserved phenomena.