Raster interrupt
An interrupt fired when the VIC-II reaches a chosen scanline, letting code change hardware registers partway down the screen.
On the Commodore 64 you nominally get eight sprites and one set of colours per frame. Programming an interrupt on a specific raster line lets you swap the background colour, move sprites or change the scroll position halfway down — then do it again lower still. The screen then shows more than the hardware nominally allows.
The raster interrupt is the foundation of nearly every C64 demo effect: opened borders, split screens and sprite multiplexing all rest on counting raster lines exactly, often cycle by cycle around a bad line.