What to call the plane between Background and Foreground?

In a 2½-dimensional design system, where x and y are planar coordinates, but z is only a signed (&pm;) stacking index, what do you call the main or basic plane (or layer or level or ground) that lies, relative to the eye of the beholder, behind all foregrounds (z > 0) and before all backgrounds (z < 0), i.e. at z = 0?

All layers can have fully transparent and partially opaque areas and thereby act as masks to lower-level layers. They can also apply more complex filters and effects, or just contain pictures, solid colors, gradients, patterns or text.

The ____ contains the textual content that is presented in front of a pictorial composite backdrop, but is only revealed completely after all overlays have faded away.

With parallax scrolling applied, the ____ moves at the nominal, constant speed, whereas foreground and background use adaptive velocity to create the visual illusion of depth.

I would prefer a word ending with …ground, for parallelism.

Options

  • In pictorial art, the virtual space of a painting is apparently divided into background, middleground (also spelt middle-ground or middle ground) and foreground. This is an intuitive term and seems most appropriate, but I am not fully convinced, because it feels like it would possibly cover multiple layers in the middle and the most important things are probably located up front, close to the spectator, whereas they are not in the foreground(s) in my use case.
  • I would have used ground as a general term for all types or groups of layers, if Q123211 did not suggest otherwise, or as the term for what lies neither overground nor underground or neither aboveground nor belowground but aground. Perhaps I am wrong, though, and it would fit the case well.
  • main ground sounds too electric.
  • ground zero – um, nah!
  • fairground or playground, possibly also raceground and showground, are possible metaphors using terms established elsewhere.
  • base, basic plane, basic layer, base layer – These do not seem wrong, but base ground or basic ground sounds odd.
  • canvas plane, canvas layer – The physical canvas is really behind even the deepest background, but the psychological canvas may be wherever the focus is intended to be.

Related questions

  • What is a word that describes the class that "background" and "foreground" are members of? – plane
  • Are the words "backdrop" and "background" interchangeable? – not quite in general, but yes in the intended context

Focal-ground seems appropriate if you wish to stick with the -ground motif. It is the area of greatest focus to the viewer.

In photography we'd typically call that area the focal field. This comes up in discussions of Depth of Field. DOF is the distance between the nearest and farthest objects that are in focus in an image.