Bertrand Russel's definitions

falsifiable

Falsifiable is a quality that theories have. It means that the theory can be tested and therefore it can be found whether it is true or false. Some theories are not falsifiable. See the body below for an example of an unfalsifiable theory.

If I had a theory that all natural laws were implemented by little invisible elves it would not be falsifiable because the little invisible elves would be undetectable. Although immune to disproof, my theory, in the absence of positive evidence for its validity, would be considered highly unlikely and, according to Occam's Razor, since little invisible elves are not a necessary assumption in order to explain the natural phenomenon we observe (we could just assume that natural laws carry themselves out, or that they are just part of the dynamics of nature) we must abandon the little invisible elves theory of natural mechanics.

by Bertrand Russel December 3, 2006