• 1something that is difficult or impossible to understand or explain:the mysteries of outer spacewhat happened after he left home that day remains a mystery
• [mass noun] secrecy or obscurity:much of her past is shrouded in mystery
• a person or thing whose identity or nature is puzzling or unknown:‘He’s a bit of a mystery,’ said Nina[as modifier]:a mystery guest
• 2a novel, play, or film dealing with a puzzling crime, especially a murder:the 1920s murder mystery, The Ghost Train