The act of auto-genital stimulation to climax in the absence of compelling visual stimuli such as videos or graphic images. Often this is accomplished through accessing a spank bank or mental storehouse of sexual, or soon to be sexual memories. It also applies where inadequate visual material (e.g. Sears catalog, high school yearbook, partially scrambled cable porn) is being used as a primer to aid intoxicated masturbation.
My laptop battery died while I was at my parents house so I had to break out the local paper and flip to the girls soccer recap for some imagination masturbation.