This term refers to the kind of dimensionality (i.e., 2D, 3D) that can be created using multiplane animation.

Since a layer in such animation can lie in front of one cel (or plane), or in back of another layer, the resulting effect is of a three dimensional world. This is a limited 3D world, however, because the layers are fixed in relation to each other. For this reason, multiplane animation is referred to as 2-1/2 dimensions. It is a very useful technique, however, even for computer graphics, because by ordering the layers in the way a painter does, you can save the computer the need to compare objects that are in different layers (that is, compare them for purposes of hidden surface removal).