The original organism decays leaving an imprint and empty space is called a mold.

Here's why:

* Fossil Formation: Fossils form when an organism dies and is buried in sediment. Over time, the organism's soft tissues decay, but the harder parts like bones, teeth, or shells can be preserved.

* Mold Formation: If the organism decays completely, it leaves an imprint in the surrounding sediment. This imprint is called a mold.

* Cast Formation: Sometimes, minerals from the surrounding sediment fill the mold, creating a three-dimensional replica of the organism called a cast.

So, the original organism is gone, but the mold preserves its shape and size.