ID X.

 X is the best known name given to an unidentified serial killer active in the largely impoverished areas in and around the Whitechapel district of London in 1888. The name originated in a letter written by someone claiming to be the murderer that was widely disseminated in the media. Within the crime case files as well as contemporaneous journalistic accounts the killer was called "the Whitechapel Murderer" as well as "Leather Apron".

Attacks ascribed to X typically involved female prostitutes who lived and worked in the slums of London and whose throats were cut prior to abdominal mutilations. The removal of internal organs from at least three of the victims led to proposals that their killer possessed anatomical or surgical knowledge.

Mainly because of the extraordinarily brutal character of the murders, and because of media treatment of the events, the public came increasingly to believe in a single serial killer known as "X".