Developing and using models explicitly demands spatial reasoning to comprehend. In many cases, the scale of a model (solar system, cell, atom) is manipulated to ease understanding. We also use models to explain geologic phenomena such as tectonic plate movement. In this case, the learner has to mentally "see" the movement of the plates.
The act of mentally visualization is spatial thinking or reasoning in its purest form. A more thorough definition was developed by Diana Sinton.
An ability to visualize and interpret location, position, distance, direction, relationships, movement, and change over space.
She just published a wonderful book that offers an easy introduction to the topic called "The People's Guide to Spatial Thinking".