Dont expect a universal definition with specifics. Been tried. imho, as a non-admin with a big mouth, if there are **ANY** one-way structures, walls or terrain you can see (or more specifically not see when you should) then you are glitching. Even if its far away across the map or just one small wall or object. That includes being on a roof you can get to with some special actions. Doesnt matter whether or not you can be killed. The mapper did not intend for anyone to be there so did not include things players would never see from that angle. Use that as a starting point. But it is not comprehensive.
The only exceptions are the air support streaks because most maps were never designed to be see from the air.