We planners are such bureaucrats, always using some agency definition of terms. But there are "cities," like say Wichita, KS, where there is precious little urbanity compared to small towns like, say, Aspen.
I have always favored a definition that is not based on density or even on use per se, but on how inhabitants make their livings. If a large number of inhabitants make their livings in agriculture, mining, or the timber industry, that place is going to be rural in character, even if it gets pretty large.
This eliminates many, if not most, places where folks say they are "rural," and would, in my mind, be therapeutic, since people tend to justify all sorts of nonsense by saying they are rural. One could then use the far more accurate "exurban" for low density places where most people commute to work in an urban place.
The questions about this approach are 1) whether a resort community can be rural? and 2) whether a retirment community can be rural? Tourism and retirement are both urban and rural activities, but since the money usually comes from an urban area, I am not sure these types of communities are rural in a meaningful sense.
I am very interested in what others think.