What’s a Mystery Novel?

I’m currently a judge for a mystery novel contest and have been thinking about the question of what is a mystery novel?

In one sense, most novels are mystery novels in the sense that there is something that we as readers try to figure out as we read. This is the broadest definition. The more restrictive definition is a novel that includes a crime, most often murder. There are also books of other genres such as fantasy or romance that have mystery elements. What do you think?