To the best of my understanding, a "dinker" is pretty much any wild type ball python that looks different than most.

Some have lighter or darker coloration, some have unusual patterns or lack of patterns, some have back stripes, or completely dark backs, etc.

The term "dinker" means it is an odd looking snake that has not yet been proven out by breeding. Most never prove out, but some do. Lots of breeders enjoy toying around with their unusual snakes just to see what happens.

I myself have a lovely "dinker" male, whose back pattern is vaguely clownish and whom I hope may produce some nicely reduced pattern babies.

Gale