A theory in science is very different from the use of the word in daily life. Colloquially, a theory is an unsubstantiated claim about why something happens, little more than a guess or an idea. In science, this kind of speculation does not reach the status of theory, but is instead called a hypothesis.A scientific theory is actually quite the opposite of its common counterpart. A scientific theory is the most reliable and rigorous form of scientific knowledge.
A scientific theory starts out as a hypothesis, simply a possible explanation for why a phenomenon occurs. To be elevated to a theory, a hypothesis must be consistent with previous experimental results, make testable, repeatable predictions, and be able to be proven false through experimentation. As more data is gathered, a theory can be modified or rejected if it does not fit observations. Because of the rigorous requirements to become a theory and the relative ease with which one can be shown to be false, accepted theories are some of the most powerful and comprehensive results of scientific inquiry.
What about scientific laws? Laws are fundamentally different from theories. A law is descriptive; it describes what happens in nature, and is sometimes stated as an equation. However, a law does not attempt to explain why things happen. Theories are wider in scope, and may contain several laws within them in order to explain observations.
Calling something “just a theory” in science is misleading. Being called a theory is not disparaging; a successful theory has withstood the trials of the scientific method, and stands proudly as the closest thing to proven that anything in science ever can be. So when someone tries to refute a scientific claim by saying it’s “just a theory,” remember that if anything that strengthens your argument, and be sure to explain to them why that is.