Plain English Meaning
Trying to start an engine again after it has quit while you are still flying. The pilot follows a checklist to get fuel, spark, and air working together so the engine catches and runs again.
Full Definition
The procedure of attempting to restore power to an engine that has stopped running while the aircraft is airborne, typically by re-establishing fuel flow, ignition, and proper engine configuration in flight. In single-engine aircraft, a successful restart converts an emergency landing situation into a normal flight; in multi-engine aircraft, it restores lost and .