Definition of jump scare:
(n) :
(narratology) The technique, typically used in horror films and video games, of having something occur suddenly and without warning to frighten the audience.
(n) :
A fright caused by this.
(n) :
(humorous, by extension) The sudden arrival or advent of something unpleasant.
(v) :
(transitive) To frighten (a film audience or video game player, etc.) by means of something that happens suddenly without warning.