A smash cut is a technique in film and other moving visual media where one scene abruptly cuts to another without transition, usually meant to startle the audience. To this end, the smash cut usually occurs at a crucial moment in a scene where a cut would not be expected. To heighten the impact of the cut, a disparity in the type of scene on either side of the cut is often present, going from a fast-paced frenzied scene to a tranquil one, or going from a pleasant scene to a tense one, for example.
An example of a clichéd smash cut is in a murder scene; a knife is raised, and thrust down, with a smash cut to a more peaceful locale (a birthday party, for example) right when impact is expected. Smash cuts are often used when a character wakes up from a nightmare, to simulate the jarring nature of that event. Smash cutting can also be used to comedic effect; for example, directly after a prediction is made, cutting to the future showing the prediction to have been humorously, and often outlandishly wrong.
It's worth observing however that when writers write scripts for film and television, writers rarely ever write in smash cuts because there is no edit physically faster than an actual cut itself, and using it therefore becomes redundant. Besides a bad writing technique to write in camera directions and edits if someone else is directing it (where a director is in his or her right to ignore the writer cues anyway), writers tend not to write these types of cuts unless there is a specific reason to do so. Writer Russell T Davies was known to write in these types of cuts in his Doctor Who teleplays, in order to add a dramatic edge for the director to work with.