There is no definition of "puppy mill". To animal rights extremists like PETA and HSUS, if you have the best cared for dogs in the world and breed them, they can label you a "puppy mill" and steal all your dogs, pose for some media shots, go dump the animals at several shelters that don't have the funding to care for them... and go on their merry way.
Some people will label EVERY single breeder of dogs as either a "puppy mill" or a "backyard breeder". No matter what. The media and the extremists have brainwashed everyone in the US that any person breeding dogs is a "bad" person who only cares about money, abuses and neglects the dogs and they are the sole reason that the cute puppy is getting killed in a shelter. It's not true. It's never been true.
If someone doesn't take care of their animals, they are "neglecting" them. If they mistreat them, they are "abusing" them. It doesn't matter if they are breeders, private owners, rescues or shelters. It is not more wrong to abuse a single ball python because you are bored or think snakes have no feelings.... than it is to starve a kennel full of dogs... than it is to beat a horse to death. The reason you own the animal doesn't matter as to whether you are abusing it or taking proper care of it.