THE definition of BYB can be found here:http://www.answers.com/topic/backyard-breeder
Although many interesting ideas have been posted, I believe that there is some confusion regarding the difference between BYBs and irresponsible breeders. Opinions and additional criteria have been added to THE definition of a BYB. It seems many of you see no difference between an irresponsible breeder and a BYB. I am also concerned that someone may think an ethical hobby breeder is a BYB.
Honestly, I can tell you that my involvment with breed rescue has been an eye opener. With some breeds, there are more rescues that come from show breeders than others. I can also tell you that many breeders on parent club breed lists breed dysplastic dogs, breed brother/sister, and mother/son or father/daughter. Some of these breeders are board members of the AKC parent breed club. So, I guess for some breeds that really throws out the breed club breeders lists.
Before I became a dentist, my degree was in genetics. I did genetic research, human genetic research. We were attempting to find markers for various forms of cancer. Having listened to "supposed experts" who show and breed, I can tell you with absolute certainty, many of these people have jumped on the latest opinion trend in breeding and have damaged the breed as whole.
With that said... my opinion regarding "Responsible Breeding" is an individual who is willing to be financially responsibile for the dogs they breed and their progeny for the life of those dogs. Keep in mind even with vast research in geneology of lines, health clearances, etc. There will still be the potential for genetic defects. There is no "perfect" set of rules to follow or way to avoid it.
Additionally, don't forget about the vast number of "Irresponsible pet owners." These people may not follow through with basic care, feeding, socialization, and/or training. This IMO is just as bad as the "Irresponsible breeder," and at times seems worse than any BYB.