whodunit

As a noun whodunit

is a novel or drama concerning a crime (usually a murder) in which a detective follows clues to determine the perpetrator.

As an adjective undefined is

lacking a definition or value.

As nouns the difference between whodunit and mystery

is that whodunit is a novel or drama concerning a crime (usually a murder) in which a detective follows clues to determine the perpetrator while mystery is something secret or unexplainable; an unknown.