What Exactly is a Mystery?
The meanings of the terms Mystery, Suspense, and Thriller are often used interchangeably, but can also be confusing. A helpful explanation comes from the Cluelass website: http://cluelass.com.
"There are no hard-and-fast rules, but Mystery and detective fiction emphasizes the puzzle aspect of the plot, with clues and "fair play" for the reader - i.e. the readers should be able to determine the solution at the same time as the protagonist.