Why is a diesel engine also called an self-ignition engine?

A diesel engine is called a self-ignition engine because, during its operation, air is first highly compressed in the cylinder, making it very hot. When diesel fuel is injected into this hot, compressed air, it ignites automatically without the need for a spark plug. This self-ignition process is what sets diesel engines apart from gasoline engines, which require an ignition spark to start combustion. So, the name comes from the fact that ignition happens by itself due to heat from compression, not from a spark.