"A technique that allows the light scattered from an object to be recorded and later reconstructed so that when an imaging system (a camera or an eye) is placed in the reconstructed beam, an image of the object will be seen even when the object is no longer present."
"When a photograph is cut in pieces, each piece shows only part of the scene. When a hologram is cut in pieces, the whole scene can still be seen in each piece. Think of viewing a street outside your house through a 4ft x 4ft window, and then through a 2ft x 2ft window; You can see the same things through the smaller window, but you can see more at once through the 4ft window and you may need to change your viewing position to see everything in the smaller window."
Holograms are really horrible things, but the idea is pretty spectacular.
See Wikipedia page on Holography.