A secret identity is an element of fiction wherein a character develops a separate persona (usually adopting a pseudonym), while keeping their true identity hidden. The character also may wear a disguise (ranging from makeup or a mask, to a complete costume). A character may have several types of secret identities simultaneously (such as adopted names, pen names, undercover identities, and crime fighting codenames).

The use of secret identities dates back to the early 20th century with characters such as the Scarlet Pimpernel, the Lone Ranger, and Zorro. Starting in the 1930s, the concept of a crime-fighters, superheroes, and vigilantes (and their adversaries) adopting secret identities became more widespread in dime novels, pulp magazines, comic books, old-time radio dramas, movie serials, and other popular fiction. Such characters remain popular to this day.

The artistic purpose of the secret identity on the part of the writers is that it allows the characters to have ordinary lives which can allow for human drama as well as create tension with the effort needed to preserve the secret. This can include challenges such as throwing off the suspicions of associates who suspect, and the need to quickly improvise means to get out of sight to change identities. And superhuman characters may benefit from an [person] aspect through having a secret identity, giving them a sympathetic link to their audience. For example, Captain Marvel's secret identity is a boy named Billy Batson - a deliberate attempt to play on the daydreams of a young readership.^{[citation needed]}

"Jack the Ripper" is the best-known name given to an unidentified serial killer who was 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 disseminated in the media. The letter is widely believed to have been a hoax, and may have been written by a journalist in a deliberate attempt to heighten interest in the story. Other nicknames used for the killer at the time were "The Whitechapel Murderer" and "Leather Apron".

Attacks ascribed to the Ripper typically involved female prostitutes from the slums 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. Rumours that the murders were connected intensified in September and October 1888, and letters from a writer or writers purporting to be the murderer were received by media outlets and Scotland Yard. The "From Hell" letter, received by George Lusk of the Whitechapel Vigilance Committee, included half of a preserved human kidney, supposedly from one of the victims. 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 "Jack the Ripper".