I doubt there's an official objective definition we can use - but the concept is extremely trivial to understand.
That imho is highly debatable.
In a broad context of science, I'd even go so far to say that without clear definitions, noone can understand anything, because you wouldn't even know what you (don't) understand.
One game that comes to my mind, where I think it's difficult to agree if it has 3D (movement) or not, is Neverwinter Nights 2. On
first glance of course it's 3D. But you need to take a closer look. When you build maps with its toolset, you can place objects in 3 dimensions. So ok, still 3D, 3 internally modeled dimensions.
But when it comes to player (or creature) movement, it's a little less obvious. Although you can model a 3 dimensional height map, with small hills and stuff, the walkmesh modeling (where the creatures may actually walk) has the limitation that you can't have a walkable area below or overhead an existing walkable area. More specifically, you cant't build a bridge that creatures can cross and that can be walked below. Additionally it's not possible for creatures (including the player chars) to fly, levitate or jump.
So at any given time, any creatures position in the 3D environment can be specified by 2 values, in 2 dimensions.
So everything looks 3D, objects positions are modeled in 3D… but it is really a 3D game? It doesn't have full 3D movement… so?
I hope you get where I'm going. The devil is in the detail.
And if you don't get your detailed definitions right, you will discuss in circles, each one talking about different things.