What does it mean to have a theory in theoretical physics?

A theory is a (tentative) description of known phenomena which makes further, verifiable predictions of phenomena. These predictions are tested and they too are supportewd by the evidence.

Without the solid backing of accurate data on the right phenomena, the description is a hypothesis.

I think string theory is called string theory because it sounds better than “string hypothesis” or “string I’m-not-crazy-no-seriously-just-listen-for-a-minute”.

Laws describe. Theories explain. This is a good article on the subject. Both are considered true, and theories never graduate to laws once they’re proven (a very common misconception). I’m a bit hazy on why string theory is named as it is, since it is more of a hypothesis than a theory. I’m guessing @the100thmonkey‘s explanation is probably spot on.

To the layman, theory is an idea that some guy pulled out of the air. Obviously, a different person will pull a different idea out of the air, so now there are competing “theories”. This causes a lot of misconceptions of what a scientific theory actually is (see @the100thmonkey ‘s comment).

There is some controversy with regard to string theory, with some scientists complaining that it has not been verified and contending that it is not capable of verification. The Trouble With Physics

String theory shouldn’t be called a theory.

A good rule of thumb: if something is called “Theory of X,” it’s probably a theory in the scientific sense of “hypothesis proven true.” Examples: theory of relativity, theory of evolution.

But if something is called “X theory” then it might just be in the colloquial sense and would be more accurately described as a hypothesis.

And science really needs to get its shit together on its use of the term theory, because too many people attack scientific ideas on the basis of “it’s just a theory, not a fact”—when for certain ideas the word theory implies they are facts.

Also, a better definition of theory in the scientific sense isn’t so much based on a single experiment or outcome. A scientific theory is supposed to explain an entire system of related facts and observations. For example, the theory of relativity explains pretty much all of our observations about macro-scale motion, gravity, space, and time. The theory of evolution explains all of our observations about the diversity of life on Earth and how life changes.

Theories can change as the systems of facts and observations expand. Relativity will one day have to be reconciled with other theories in the quantum world. But in general a theory is considered “proven” in the sense of explaining and tying together the broadest possible range of observations.