Definition of "jumpscare"

I sit shaking in fear and shock from the horrible jumpscare.

2011 January 27, [ste parker], “Re: BA Alan Wake”, in uk.games.video.misc‎ (Usenet)

Do you like jumpscares? Well then, here's your movie!

2011, Justin Mancini, "'Paranormal Activity 3' delivers more of the same scares", The Signal (College of New Jersey), Volume 135, Number 8, 20 October 2011, page 21

It seemed that for every legitimate moment of tension and terror caused by Wan's brilliant style of camerawork (of which there surprisingly weren't many), there was a cheap jumpscare that just made you think: “Did they really just do that to me?”

2013, Patrick Shanahan, "Insidious Chapter 2 Falls Flat", The Record (Saint Louis Priory School), Volume 44, Issue 5, 27 September 2013, page 6