An especially sharp transition. This style of cut is usually used to convey destruction or quick emotional changes.
For example: If I were writing a horror movie but wanted to lighten the gore at the beginning, I might have the first victim trip and fall. The killer enters the forest clearing, taking a moment to savor this death. The victim shakes her head, as if begging for the killer to change his mind. But no, he closes in, a black cloaked arm raising the knife into the air. The knife catches the moonlight for just a moment before it races downwards.
SMASH CUT TO :
EXT. WOODLAND HIGH SCHOOL COURTYARD - DAY
It's a bright and beautiful morning and kids wander the courtyard on their way to class or to meet friends. And the students discuss the end of this example.

The sudden shift from a dark forest to a bright schoolyard on the first stab would convey the distress of the murder without showing it. For another example of a smash cut, see the transition to L.A. in Barton Fink.

Note: this transition is often a director's choice. As a writer, use this sparingly if at all. Many script readers find this term unprofessional.