Theory

Existant

A theory is a coherent set of mechanisms that are assumed to be working in a certain domain, thus creating predictable effects. Sometimes one uses 'causal relationships' instead of mechanisms, but I prefer the latter, because causation has some vagueness to it, if the underlying mechanism is unknown.

Furthermore a theory can be tested by empirical evidence. When it is true it is said to be verified, and when it is false it is said to be falsified. Sometimes theories are still not testable (like string-theory), which doesn't make them false, of course, just untested.

Anyway wikipedia can elaborate on the term of theory.