I think the visualization definition has been answered on here quite well, I couldn't have described it as well, myself, but I know of some ways to increase your imagination skills.

You could try imagining to music, when I listen to a very fast paced song, I will usually imagine a scenario where there's a person fighting for either himself or others, depending on the song, it's just what I enjoy visualizing.

There's also reading, I read every other day or so, it helps me quite a bit, with all of the descriptions of actions, characters, looks, etc.

Hope I helped.