Sherlock AU is the name given to the scenario where the Big Four meet famous detective Sherlock Holmes, usually the incarnation from BBC's hit show Sherlock starring Benedict Cumberbatch and Martin Freeman in the lead roles. Much like Hogwarts AU and Storybrooke AU, it is often considered a spinoff of the Modern AU and Crime AU. Although it is a relatively small AU, it does have several loyal followers.
With Sherlock making a comeback into modern society, it's no surprise that people wanted to add him - despite the fact that he is live action - to the fandom. Usually, the Four encounter Sherlock and John Watson in modern day London, where he solves mysteries for them. In other cases, they time jump or are already living in 19th century London.