Crime fiction, detective story, murder mystery, mystery novel, and police novel: These terms all describe narratives that centre on criminal acts and especially on the investigation, either by an amateur or a professional detective, of a serious crime, generally a murder. It is usually distinguished from mainstream fiction and other genres such as historical fiction or science fiction, but the boundaries are indistinct. Crime fiction has multiple sub-genres, including detective fiction (such as the whodunit), courtroom drama, hard-boiled fiction and legal thrillers. Most crime drama focuses on crime investigation and does not feature the court room. Suspense and mystery are key elements that are nearly ubiquitous to the genre.