So, what does the phrase really mean? It depends on who you ask.
For some, a puppy mill is a dog breeding operation, either large or small, in which profit is the focus of the business and in which the well-being of the dogs is of little concern or at least secondary concern. Dogs from this type of breeding operation routinely spend their entire lives in small cages. Many of the cages are wire crates which lack solid floors and are no larger than your dishwasher. The dogs are more often than not physically injured or psychologically scarred from the conditions in which they are forced to live and they receive no veterinary care at all. They have cancers, are missing eyes and parts of limbs, their teeth are likely rotted and they may have a host of other serious health issues which go unseen. They are bred repeatedly until they are no longer capable of producing profitable puppies. This is the type of business which is routinely the subject of media reports, articles written by journalists and solicitations for donations made by large animal welfare organizations.
There are some who go one step further and fault all dog breeding operations, even those by responsible breeders who only breed dogs periodically for families, for breed competitions or for use in some service capacity. These people proclaim that we should not "breed or buy while shelter dogs die" and see all breeders as the enemy.
Looking at the words which make up the phrase, we all know what the word "puppy" means. It refers to a young dog. The word "mill" is also understood by most people. It has historically meant a building equipped with machinery for grinding grain into flour. It can also mean a factory for certain kinds of manufacture such as paper, steel, or textiles. More recently, the word has been used to describe a business or institution that dispenses products or services in an impersonal or mechanical manner, as if produced in a factory. Examples include "divorce mill" or "diploma mill."
I have no issue whatsoever with using the phrase puppy mill to describe commercial dog breeding operations. I would not call my dentist (who breeds Black Russian Terriers for dog competitions) a puppy miller because she is a responsible breeder who has had very few litters of puppies in the almost two decades we have known each other. Her breeding of Black Russian Terriers is to perpetuate breed standards and her dogs have competed in major competitions. When looking beyond these small-time breeding operations like hers, I focus on the meanings of the individual words. Puppies are being produced. They are being produced in a factory or mill-like manner for profit. So for me, a puppy mill is any industry which produces dogs on a regular basis in any volume as a money-making venture.
I appreciate the fact that some dog breeders are not happy with use of the phrase puppy mill. I presume that people who raise dogs as part of a multi-generation family business and who work hard to care for the dogs don't like being labeled in a way which causes them to be perceived in the same way as are those who see dogs as inventory, caring little for the well-being of the "breeder stock." To those people who feel offended, I would say this. The phrase is used to describe behavior related to a chosen profession producing dogs for profit. If you find the phrase offensive, that is because you are interpreting it personally, as if it is a personal attack on you as an individual. It is not. It is a reference to your mass production of dogs in an industrial or mill-like manner. If yours is an operation about which you are proud and which bears no resemblance to the types of horrid operations we read about in articles or see in programs on Animal Planet or see as the subject of marketing campaigns to solicit money to stop the objectification and abuse of dogs, by all means be proud of what you are doing.
Advocacy is about speaking out about things we want to change in our society. When it comes to animal welfare advocacy, we speak out on behalf of the animals who cannot speak for themselves. I think we need to careful to not blame the messenger for the fact that the message was necessary in the first place. The animal-loving public is becoming increasingly aware of, and alarmed by, what takes place in the commercial dog breeding industry. We don’t hate those who breed dogs in mass quantities as people. We disapprove of their behavior. And we want it to change or stop. Using the phrase "puppy mill" to help educate the public serves a purpose as our use of language is molded and shaped to fit our culture. It is a tool to communicate a concept.