[Edited by Dhampy, 9/13/2009 5:41:36 PM]
Although to be practical, probably the only way to meaningfully define them is by comparison.
There is light in a pitch black room. The human body gives off ambient light and so does our clothing which is washed in phosphorescent chemicals.
The most video-game like massacre of this type was in 1964 in Germany when a guy attacked a school with a flamethrower, a medieval mace and a lance.
Had that happened today, every pundit and politician would be blaming video games. Lawsuits would put game companies out of business. The backlash would be of epic scale.
Was he compelled to do so by video games? Of course not, there weren't video games in 1966.