Very well, then let's try this. Again, keeping in mind that there is no set definition, this is just mine.
A roadside zoo is an unaccredited, privately-owned, for-profit facility that exhibits non-domestic animals and has demonstrably poor welfare (in the US often determined by frequent USDA citations) and/or at least half of their enclosures substandard for the species housed in them.
I find the unaccredited/private part to be important, as a lack of oversight is a major factor of roadsides and why they are able to be so poor quality. Not every "bad" zoo meets this definition, just as not every "good" facility is accredited.
I think criticism about AZA's accreditation process is not entirely justified. Yes, there are lots of questions in the application which do not directly relate to animal care. It's possible that a facility could satisfy those requirements and still bomb the rest. The other sections have their importance, too - they are meant to ensure that the facility is run in a safe, transparent, economically sustainable manner in compliance with local non-animal laws and regulations. It does a zoo very little good to have good animal care but always be teetering on the edge of collapse, or have chaotic leadership fighting and turnover jeopardizing operations.
