Building up suspense without boring the audience is not easy. Whatever is a director to do? Jump scares to the rescue! Everyone knows what a jump scare is: it's the horror technique of having something happen unexpectedly and suddenly (usually something popping up out of nowhere). It's frequently accompanied by an equally sudden loud noise to go with it, and often happens after a period of tension-building to ramp up the nerves of both the character(s) and audience and make the scare that much more effective for both.
The classic film example uses a loud brass horn, or a woman's scream. Jump Scares follow the law of diminishing returns. They're effective when accompanied by a slow build up of suspense, but too many will turn them into more of a nuisance than a genuine scare, as demonstrated by the many, many horror Video Game examples in which the trope is overused to the point of predictable, if occasionally still somewhat startling, banality, by creators with little grasp of what makes horror scary. The film critic Nigel Floyd refers to the occurrence of this in films as "cattle prod cinema" — you're not being scared, just jolted. Conversely, of course, when creators who do understand horror make use of the trope, they do so to extremely good effect, to such an extent that it works even on a repeat player who knows it's coming, to say nothing of a newbie with no reason to predict it.
Sometimes overlaps with Mirror Scare, Screamer Trailer, Spring-Loaded Corpse, Peek-a-Boo Corpse, Take a Moment to Catch Your Death, Nightmare Face, Scare Chord, Surprise Creepy, and Last Note Nightmare.
When used on its own in a Web Original, it can be known as a screamer or a prank. You know, the videos that encourage you to turn up the stereo and/or examine the picture closely. Related to Shock Site, where the startlement and horror arises from unexpectedly viewing a disturbing image. Contrast Sudden Soundtrack Stop, where the sudden lack of noise can be used for horror.
No examples, please. We're just defining the term. If we did include examples, chances are we'd be sued for accidentally scaring some of our more delicate readers into the next life, if you catch our drift.