This describes an involuntary movement made by the cat after perception of a stimulus, either environmental or social, which is threatening, perceived as threatening or has simply given the cat a surprise. The movement is quick and brief and may involve displacement of the entire body (for example a jump) or a head movement only. During a startle response orientation of the cat’s senses (eyes, ears and possibly whiskers) will be towards the stimulus. A startle is frequently followed by a run or a freeze response.
The following video illustrates a startle to a noise and a run (or flee) response. Note the flee response does not last long suggesting the perception of the startle stimulus is not overly threatening. A more threatening stimulus would have a larger flee distance (and may also lead to a quicker flee).
In the following video, the cat startles to the sound of the camera. Note in this case he freezes instead of fleeing. Also note the vertical tail switch which is believed to relate to an increase in arousal level.