Citylaw has summed it up nicely, but just to expand on a couple of points:
So the definition of "good" is ABA?
I would say yes.
The entire point of ABA accreditation is to ensure that law schools which carry the imprimatur have met or exceeded a series of standards. This creates a predictable, reliable system of legal education which doesn't really differ much between schools.
The standards for accreditation require that the school not only offer an approved curriculum, but that it be financially solvent, have faculty hiring practices in place, offer academic support, meet bar passage standards, and about a hundred other criteria.
In other words, ABA accreditation tells the consumer that they will receive a "good" legal education subject to objectively verifiable criteria. And contrary to popular belief, it's not that easy to obtain ABA accreditation. It takes a lot of money, a lot of dedication on the part of the institution, and years of time. ABA site teams visit every year during the application period and report back to the committee on legal education. They stay on campus for about a week at a time interviewing students and faculty, going over finances and admission standards, sitting on classes and evaluating the academic program.
Prestige is an entirely different matter. Some schools have it and some don't, and it has nothing to do with ABA accreditation. I guarantee that Harvard and Yale were considered elite long before ABA accreditation was conferred.