I dont have a good, concise working definition but this is my take on it. Some might say heritage means a breed original to this country, but I would say it's more than that. I would say heritage breed preservation does not mean indiscriminate matings in large numbers of a breed with the intent of merely making more birds, regardless of the standards originally set forth. They are not just "chicken mill" hatchery stock, but more closely related to the original breed, not diluted as it were, but bred to a prescribed standard. For instance, the Rhode Island Red is a great common example. How many see the rich, dark, black cherry/mahogany color in hatchery birds, the proper shape and size, etc? You rarely do, if at all. Today's hatchery RIRs are bred for egg production, and not much else.
I hope I didn't butcher that too much, but I'm sure seriousbill and NYReds and some others might be able to tighten up my definition a bit.