William DeAndrea defines mystery as "the literature of crime--of robbery, chicanery, murder, and worse" (Encyclopedia Mysteriosa, p. ix).
Traditionally, that crime is murder, and mystery is the genre of literature that portrays the detective's process of discovering who the killer is. The detective in question may be either a police officer (in which case the novel is called a police procedural), a private investigator (PI), or an amateur (e.g., Miss Marple or Jessica Fletcher). There are other types of mysteries in addition to the police procedural, most notably the cozy, which comes from the British tradition, the hard-boiled, and the noir.
A mystery is technically different from a thriller. In the mystery, the killer is unknown to both the detective and the reader, and both the detective and the reader proceed through the story together, with the reader trying to figure out the killer's identity along with the detective. In a thriller, the reader knows from early in the story who the killer is; the reader then watches the detective at work, hoping that the detective will find and subdue the killer before the killer kills again. This approach is becoming increasingly popular in modern novels, which often present the story in alternating chapters from the detective's and the killer's points of view.
Like other genres, mystery has its conventions, its rules of the game. A good statement of those conventions is Raymond Chandler's ten commandments for the detective novel, below. A good mystery author plays fair with the reader by honoring these conventions.
© 2007 by Mary Daniels Brown
ten commandments for the detective novel
It must be credibly motivated, both as to the original situation and the denouement.
It must be technically sound as to the methods of murder and detection.
It must be realistic in character, setting, and atmosphere. It must be about real people in a real world.
It must have a sound story value apart from the mystery element; i.e., the investigation itself must be an adventure worth reading.
It must have enough essential simplicity to be explained easily when the time comes.
It must baffle a reasonably intelligent reader.
The solution must seem inevitable once revealed.
It must not try to do everything at once. If it is a puzzle story operating in a rather cool, reasonable atmosphere, it cannot also be a violent adventure or a passionate romance.
It must punish the criminal in one way or another, not necessarily by operation of the law . . . . If the detective fails to resolve the consequences of the crime, the story is an unresolved chord and leaves irritation behind it.
It must be honest with the reader.
[From The Book of Literary Lists (1987) by Nicholas Parsons, page 129]