noun, plural: theories
(2) An expectation of what would happen, excluding unforeseen circumstances. For instance, the theory that criminals usually return to the scene of the crime.
(science) A well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment.
The common use of the term theory implies speculation or assumption that has not been verified or has limited proofs. In science, an unproved idea or a mere theoretical speculation is regarded as a hypothesis rather than a scientific theory. However, in science, a theory is a well-substantiated explanation or a set of statements that have been confirmed over the course of many independent experiments. In comparison, laws explain things but don't describe them, whereas theories describe why we see a set of observations. E.g., Newton's Law of Gravity describes how gravity works but not why there is gravity. Darwin's Theory of Evolution by Natural Selection tells us why the observations about life on this planet are as we see them. Both scientific theories and laws are based on facts and are accepted to be true by the scientific community. And both are used to make predictions of future events.