A fossil formed when an animal, plant, or other organism dies and is covered by sediment, its flesh decays and bones deteriorate due to chemical reactions, and a cavity remains below the ground surface; the cavity's interior surface is an impression in the general form of the original organism's exterior surface.
noun
A fossil formed when sediment fills the inside or covers the outside of a dead organism and the organism's remains do not persist, leaving just the shape and texture of the rock to indicate the organic material that was there.