A volumetric 3D display is a display that exists in more than 2-dimensions – it literally has volume. Here’s an early version that for now is mostly 2D but a future version will include a volume:
It is hard to tell from the photo what is going on so I will try to explain. Here, a humidifier like device is creating a translucent wall on to which interactive video images (computer generated) are projected. The system also includes sensors that identify hand movements so that, as shown her by their marketing rep, can interact with the projected objects.
A future version (quite a ways out yet) will expand this planar version in to a volume.