Falsifiable - capable of being tested (verified or falsified) by experiment or observation.  It is a main part of the scientific method. The importance of falsifiability in science was first suggested by philosopher, Karl Popper.
This is the prime test for whether a proposition or theory can be described as scientific.
Paradoxically,that which is falsifiable, but has not yet been shown to be false, is accepted as scientific; that which is not falsifiable is not accepted as scientific in some quarters.
Note that scientific does not necessarily mean true, but only, 'true as far as current methods of observation and experiment allow'.
Until the twentieth century Newton's laws of motion were
- a) scientific and
- b) believed to be true.
Relativity, by finding, and employing, the falsifiability of Newton's laws, modified them.
Similarly creationism is not falsifiable as its proponents base the theory on human actions (the writing of the Bible) which cannot be tested by observation or experiment. This is one of the primary tests of pseudoscience.
Just because a Theory is falsifiable, it doesn't mean that it ever will be falsified, as many anti-evolutionists seem to believe. The fact that a theory is falsifiable means that it is scientific - not that it is wrong.
According Popper, theories in science must make testable predictions in order to be falsifiable. The more a theory forbids, more specific a theory becomes, thus more risky as well. When a theory that makes a risky prediction is vindicated, the result has larger impact in establishing reliability of the scientific theory .
- Popper, Karl, Falsification. New York, USA: Oxford University Press, 1999.