A diorama is a small three-dimensional portrayal of a scene, location, or space. Traditionally a diorama is a physical thing, with figurines and minature objects fabricated and arranged within a suitably sized box.
An image is tagged diorama if it depicts an actual diorama or if it is visually reminiscent of one, foremost by emphasizing three-dimensionality (via perspective or by being isometric) and by suggesting an imaginary box, via artificial, sharply defined edges to the space, as if cut by a giant knife (for example, post #3307512).
A diorama is distinct from faux figurine, in that while the latter focuses on a single character, the former focuses on either many, relatively small characters or none at all, serving as a landscape or still-life.