| in the context of photography, the skill/activity of visualizing how the picture will come out.
This sounds trivial, but it is not: even if you are not using a view camera (and thus dealing with an upside-down, left-right inverted image), the picture will be taken on film.
Film does not react to light as the human eye does: its contrast range is different, it accumulates photons (to a certain extent, and always accounting for reciprocity failure), and it does not adapt to light color temperature.
In many cases, previsualization also involves thinking about the printing step: in the Zone System, in order to have a picture just so, with a medium gray sky, pure white snow and featureless black stones you need to precisely expose film, that will be subsequently precisely developed and precisely printed.
All this precision is difficult.
Previsualization requires intimate knowledge of film behaviour. This is why professional photographers are reluctant to change film: they already know how it behaves in all light situations, and with new film they would have to go through the long, slow, learning process one again.
Of course, previsualization is a very deliberate, intellectual process (even if eventually one might do it without realizing it). IMHO, it is what distinguishes the professional photographer from the amateur. When previsualization becomes unconscious, then you have a master (or rather, a kind of master: there are many others). | 
