The common usage of the word "theory", referred to here as informal theory, refers to "a conjecture, an opinion, or a speculation. In this usage, a theory is not necessarily based on facts; in other words, it is not required to be consistent with true descriptions of reality. True descriptions of reality are more reflectively understood as statements that would be true independently of what people think about them." [W]
This should not be confused with scientific theory, which has rather more rigorous requirements.