In Diesel, the duration between the time of injection and the time of ignition is called Delay Period.

What is the Delay period in CI engine?

The diesel engine is a compression ignition system. The injector injects the fuel into the cylinder and the fuel self-ignites due to a high temperature of compressed air. But the atomized fuel from the injector is considerably colder than compressed air in the cylinder. So, after injection, it takes some time for the fuel to reach the Self-Ignition Temperature (SIT). This period of time is the Delay period or Ignition lag.