Corpsing is British theatrical slang for unintentionally breaking character by laughing. In North American TV and film this is commonly referred to as breaking and is generally categorized as a blooper. The origin of the term corpsing is unclear, but may come from (provoking an actor into) breaking character by laughing while portraying a corpse.
A significant aspect of the phenomenon is the frequently deliberate and usually benign attempts among actors to cause this in cast members. During the "Pete and Dud" sketches in the BBC comedy series Not Only... But Also, Peter Cook would deliberately ad lib in an attempt to make Dudley Moore corpse—and invariably succeeded.
Corpsing is not exclusive to the theatre. One of the most famous examples of this is on the cricket programme Test Match Special in the famous "leg-over" incident and another cricket commentary in which it was noted that "...the bowler's Holding, the batsman's Willey."
In the Monty Python film Monty Python's Life of Brian Michael Palin deliberately attempts to make background actors, who were told not to laugh at the risk of being fired, corpse during the scene in which Brian is brought in by the Centurion played by John Cleese and two of his guards. Michael himself nearly corpses in the process.