From The TV IV
Dramatic irony is a term borrowed from literary and dramatic criticism. It refers to a situation in which the audience is shown or made aware of an important fact, detail or story element of which one or more characters are unaware. In television, the best example of dramatic irony is in every episode of Columbo. Viewers would be shown a homicide in the first act, and would therefore know both the killer and his or her motive, but it would be up to the detective, Lt. Frank Columbo, to solve the case in the remaining two acts.