Raster image processor.
Software used to convert the data sent to be printed into a format that is understood by the digital printer. It normally happens in the background and is invisible to the user. It converts any fonts, effects or vectors used in the file into pixels of the correct resolution that can be imaged by the printer.
Larger printers and image setters will normally have a separate RIP on a stand alone computer, which will often include a user interface which will allow users to queue jobs and alter details like quantity and orientation.