Pattern or continuation on a sub-board that is not to be found in a database search of pro games. If the sample of games where it could occur but doesn't is large enough, one can reasonably conclude that (a) the pattern is rare in pro play and, much more significantly, (b) that there is some factor forming part of pro-level conventional wisdom that blocks this happening in high-level play.
Examples on SL include those discussed at
These all relate in some way to the conventional treatment of frameworks, and are therefore rather different from thinking about joseki innovations. In the case of joseki one can spend a great deal of time considering plays that are 'also possible'; the fact that they aren't played by pros may not be an indication that they are bad.