If you must be vague to even talk about it, then what's the point of even using the term?
What's the point of naming fictional genres? Or business processes? People still seem to manage to have discussions about them.
Being utterly vague and not being able to say anything definite means that it's so subjective, there's no baseline. There's no way to agree, and no real meaning to it.
I don't think we've been quite that vague, though.
Honestly, to me part of the issue is that the essence of what is really "sandbox" isn't really deep, and it isn't complicated.
The GM makes up a sandbox, and populates it. The PCs are dropped into it, and told they can go where they want, and try to do what they want. The GM takes no particular pains to get the PCs to go in any particular direction. When they choose to go somewhere or do something, the GM does not alter what he'd prepared to meet the PC's abilities.
That, really, is the core of it. All the rest is variable:
How much space? You can have a sandbox town, nation, continent, world, or multiverse...
How much content is pre-populated, and to what level of detail? Well, I doubt anyone's going to pre-populate every house on every street of every town on a planet. There may be some things that are generated on the spot (say, by wandering monster tables, or wandering townsfolk tables, or what have you).
Are all of the pre-populated elements active? Again, doing that for every single person and critter on a planet isn't practical. Some of the elements are not going to be specified and active - many are apt to be assumed and/or in a holding "status quo" state. Could all of them be like that? Well, I wouldn't do it that way, but it isn't like there's some official "minimum active content" or something - and it is not as if anyone has the authority to designate such.