(excerpt from http://en.wikipedia.org/wiki/Falsifiability)
Falsifiability (or "refutability") is the logical possibility that an assertion can be shown false by an observation or a physical experiment. That something is "falsifiable" does not mean it is false; rather, that if it is false, then this can be shown by observation or experiment. Falsifiability is an important concept in science and the philosophy of science. The term "Testability" is related but more specific; it means that an assertion can be falsified through experimentation alone.
Some philosophers and scientists, most notably Karl Popper, have asserted that a hypothesis, proposition, or theory is scientific only if it is falsifiable.