Volume
A three-dimensional space defined by its boundaries, or implied by the relationships between the planes that suggest it.
In practice
Volume is three-dimensional space, either solidly enclosed, like a box, or implied by planes that suggest its boundaries, like four thin posts that mark out a cube without walls. Industrial designers and architects shape volumes directly, while graphic designers suggest them with shading and perspective. A lamp made from two flat sheets slotted together crosswise can still read as a full volume because the planes imply it.
Often confused with Mass
Volume is the three-dimensional space a form occupies or implies; mass is how heavy that form looks or feels.
Try this
Build a cube from twelve straws or paper strips as edges only, with no faces, and notice it still reads as a volume.