In a theatre, the prompt corner or prompt box is the place where the prompter—usually the stage manager in the US or deputy stage manager in the UK—stands in order to coordinate the performance and to remind performers of their lines when required.   It is traditionally located at stage left.
Historically, the prompt corner was situated at stage left. Prompt side (abbreviated to PS) and opposite prompt (abbreviated to OP, sometimes called off prompt) are widely used terms for stage left and stage right.  However some theatres choose to install prompt corner in a discreet area of the auditorium. Certain theatres which locate their prompt corner on stage right would inform cast and crew that they were operating on a bastard prompt system. 
The prompt corner is usually equipped with a prompt desk to facilitate the coordination of a performance. This can vary from a small table in the wings to an elaborate installation in a dedicated booth, being equipped with all the necessary aids for the specific production and venue. The prompt desk minimally holds a carefully annotated copy of the performance script, with blocking and other stage directions and, in professional theatres: