Plain English Meaning
How long it takes a pilot to notice something has gone wrong and start doing something about it. During that short pause, the airplane is still flying on its own with no input from the pilot.
Full Definition
The interval between the moment a pilot perceives an event, such as an engine failure, and the moment a corrective control input is initiated. In single-engine engine-failure-after-takeoff scenarios, reaction time is treated as a fixed delay during which the airplane continues to decelerate and descend before the pilot lowers the nose to maintain best glide.
Context Anchor
Encountered in engine-failure-after-takeoff discussions, where even a few seconds can affect airspeed, altitude, and landing options.