Film technique used almost exclusively in horror films. It describes the use of something innocuous, albiet unexpected, as a scare. For example: the cat that jumps out of the cupboard, scaring the protagonist/victim/audience.
The technique was popularized, and hence named after, movie producer Val Lewton. Lewton is perhaps best known for the films Cat People (1942) and The Leopard Man (1943). These films were dark and atmospheric, and worked with the (most would say correct) assumption that what you can see isn't nearly as scary as what you can't.
In Cat People, at the end of a very tense scene, a bus roars into the frame, brakes squealing, scaring the pants off of anyone who watches. Hence the "bus" in "Lewton bus."