A valid scientific theory must summarize a hypothesis, be supported with a body of evidence taken from further testing and be submitted for peer review. In science, a theory is used to describe how something works, not why a scientist thinks something may work. For example, gravitational theory describes how the fact of gravity works, and evolutionary theory describes how the fact of evolution works.
Many people make the mistake of equating a vague plan or idea with a scientific theory. A proper scientific theory is first known as a hypothesis. Only after a hypothesis has been properly tested with further experimentation and then evaluated by other scientists can it be called a scientific theory.
This misunderstanding of the term often leads some to claim that solid scientific principles, such as evolutionary biology, are incorrectly dismissed as "only a theory."