Answer:
A "theory" refers to an idea which may or may not have been scientifically tested and/or well documented. The main point about a theory is that it is always open to being proven wrong.
Example: my cat is black and everyone observing it says it is black. So a theory can be stated thus: "I have a theory that my cat is black". So far, that theory has been 100% supported during repeated testing.
The Theory of Relativity concerns the structure of space, time and motion.