When is an invention really radical?

We develop a valid definition of technological radicalness which states that a successful radical invention is: (1) novel; (2) unique; and (3) has an impact on future technology. The first two criteria allow us to identify potentially radical inventions ex ante market introduction; adding the third condition, we can ex post determine if an invention served as an important change agent. Empirically testable condition selected 6 of 581 tennis racket patents granted between 1971 and 2001. Two of the identified patents - the oversized and the wide-body rackets - are considered radical inventions by industry experts. Applying our definition and operationalization would allow researchers to achieve greater generalizability across studies, avoid endogenous definitions of radicalness, and study predictors of market success for radical inventions.