Here is the ol' snippet from Wiki: the space around and between the subject(s) of an image. Negative space may be most evident when the space around a subject, and not the subject itself, forms an interesting or artistically relevant shape, and such space is occasionally used to artistic effect as the "real" subject of an image.
One of the images they used to illustrate was the popular black and white "A Goblet Or Two People Kissing" optical illusion. I shall use an old blot and a drawing someone did for it for our own illustration.
As that person saw it, the image was there in the area without the ink BECAUSE of the area used by the ink.
Kind of a grim interpretation, but then that is why I love these things. Sorry I didn't get to doodle on one of the deliciously wacky responses from yesterday's blot.
Here is today's: