Einstein was so far out there, we are just now beginning to realize what he said is truth and not a theory.
There are two meanings of "theory"...there is the colloquial meaning, such as "I have a theory as to why my favorite sports team is doing so well this year." In this context, it is a synonym for "Hypothesis" which means it is largely a guess, it may have no evidence to actually back it nor does it make testable predictions. This is the meaning that people mistakenly use when they say of some scientific theory, "but it's only a theory."
By contrast, a theory in science is the graduation of an idea based on evidence. It is a well-substantiated explanation of some aspect of the natural world that is acquired through the scientific method and repeatedly tested and confirmed through observation and experimentation. Some examples are Einstein's General Theory of Relativity, The Germ Theory of Disease, and The Theory of Evolution.
These are not mere speculation or unsubstantiated ideas, but rather they have been tested repeatedly, the predictions they make have been confirmed, and evidence all converges on them. So, when speaking of scientific theories, rather than saying it's only a theory, one should actually say, it is an accepted theory.
A scientific theory can be challenged at any time though (part of the great thing about science in that it is a self-correcting process), and in the natural sciences, there is no such thing as absolute truth. If a different explanation for some aspect of nature is put forth, and it makes its own predictions that agree with the currently known data, and is found to make further predictions that disagree with the current theory, and when investigated, the current theory fails while the new theory succeeds, then over time the new theory becomes accepted and the old theory is discarded. This is in large part what is meant by scientific progress.
A prime example of this in physics is when Einstein's theory of gravity essentially replaced that of Newton. Both theories predict that a ray of light will be bent by gravity, but they disagree on how much they will be bent by a particular gravity field. Observation of starlight passing near the sun during a solar eclipse showed that Einstein's prediction was much closer to the observed reality than was Newton's. However, Einstein's theory of gravity is a classical theory, in that it does not incorporate quantum mechanics, so we know it is incomplete...and until a quantum theory of gravity is found, we know our understanding of gravity is incomplete.
