Plain English Meaning
The instant jolt you feel when something unexpected happens — the quick body-and-mind flinch that hits before you've had time to think about what's going on.
Full Definition
An involuntary, brief physical and mental reaction to a sudden, unexpected stimulus. In aviation human factors, startle refers specifically to the short-duration reflex response that occurs in the first moments after an unexpected event, before conscious thought catches up. It is distinct from surprise, which is the longer cognitive reaction that follows.
Context Anchor
Seen in discussions of pilot response to unexpected events, such as a warning light, unusual aircraft movement, or a sudden change during takeoff or landing.
Derivation
From Middle English 'stertlen,' meaning to rush or leap suddenly. The word has always described a sudden involuntary reaction. Knowing this helps separate startle (the immediate reflex) from surprise (the slower, thinking response that comes after).