Arkham Asylum is a prison for mentally criminals in the Batman series, and it is one of the most notable places in Gotham City.
The Asylum's most notables patients are the Joker, Scarecrow, Two-Face and Poison Ivy. It also fitted for criminals with medical needs outside of ordinary prisons such as Mr. Freeze.
Originally, Arkham Asylum is used only to house genuinely mentally unwell patients having no connection to Batman, but over the course of the 1980s, a trend was established of having the majority of Batman's supervillain opponents end up at Arkham.