Definition: Apoptosis is the process of programmed cell death. It is used during early development to eliminate unwanted cells; for example, those between the fingers of a developing hand. In adults, apoptosis is used to rid the body of cells that have been damaged beyond repair. Apoptosis also plays a role in preventing cancer. If apoptosis is for some reason prevented, it can lead to uncontrolled cell division and the subsequent development of a tumor.