The prompt (sometimes prompter) in a theatre is traditionally the person who prompts or cues an actor when they forget their next line or neglect to move on the stage to where they are supposed to be situated.
Usually the task is undertaken by the stage manager, who will have a copy of the script called the prompt book. This is the most definitive version of the script for any one performance, and will contain details of all cues, with their precise timings with respect to the action on stage. This allows the prompt to direct lighting, sound and flying effects and scene changes during a show. The prompt book also often contains blocking notes, so that the prompt is always aware of the intended positions and movements of all the actors on stage at any given time.
The prompt is located on the stage, in the prompt corner.