An impostor, or imposter, is a person who pretends to be somebody else, often to try to gain financial or social advantages through social engineering, but just as often for purposes of espionage or law enforcement.
Pretenders for various thrones used to be common. Numerous men claimed they were Dauphin, the heir to the French throne who disappeared during the French Revolution, and there were three false Dimitris who were serious pretenders for the throne of Russia.
Very daring impostors may pretend to be someone else who really exists, although fast news media has made this rather difficult. Usually, they just misrepresent their financial status, educational status, social status, family background or in some cases, their gender. Impostors are usually aware of not being who they say they are. However, there are borderline cases who may end up believing their own tall tales. People may make false claims about their past or background without being full-blown impostors; non-existent military service seems common.
Many temporary impostors are criminals who maintain the façade for a time of a caper, to defraud their victims (like Wilhelm Voigt).
Others, like US prankster Joey Skaggs, do it as a prank or to make a point of some kind. The latter usually reveal the truth sooner or later. Some, like John Howard Griffin, have adopted other identity for purposes of research, investigation or experiment. Note that although impostors usually misrepresent their background, their intentions may not be criminal as such. They may wish to start afresh with a new identity or "go native"; i.e. adopt the identity and customs of other people.
Sometimes, women have masqueraded as men, to obtain privileges only men can have, or to work in male-dominated professions. Some have fought as men, at least in Napoleonic Wars and the American Civil War. Sometimes, an organization or individual, who has been fooled, keeps quiet to avoid the embarrassment, and, therefore, allows the impostor to try the same thing elsewhere.
Of course, the most successful impostors are those whose duplicity is never revealed. We know nothing about them.
Lobsang Rampa, who claimed to be a deceased TibetanLama possessing the body of Cyril Hoskins and wrote a number of books based on that premise.
Aleksey Vayner, star of the "Impossible is Nothing" video résumé, who pretended to be the CEO of a capital management company and a charity; to lift 495 lbs.; to serve a tennis ball at 140 mph; and, to break bricks with his hands, in order to gain an entry level job at UBS.
Charles Stopford, an American man who has claimed to be an English nobleman since 1983, using the name of a dead infant.
Count Dante is the real name of John Keehan. Many don't recognize his rationale for assuming the title, and allegedly rightful, name of Spanish nobility. In his campaign to promote his system of martial arts, he also claimed victories in various secret deathmatches in Asia, and mercenary activity in Cuba, none of which carried documented proof.
Mary Carleton who was, amongst other things, a false princess and bigamist.
Marvin Hewitt, who became a university professor without real credentials.
James Hogue, who most famously entered Princeton University by posing as a self-taught orphan.
Marilee Jones, Dean of Admissions at MIT and a best selling author who claimed advanced degrees in science fields. After ten years in the post, she was revealed to have only a high school diploma.
Brian MacKinnon, who went back to being a teenager in order to re-enter medical school.
Azia Kim, who posed as a Stanford University student for eight months, before finally being caught.
Barry Breman has entered multiple sporting events pretending to be an MLB umpire, an NLB all star, and Dallas Cowboy Cheerleader, among other things.
Women who lived as men
Many women in history, who may not have been transgender, have presented themselves as men in order to advance in typically male-dominated fields. See also: Crossdressing during wartime.
James Barry, who successfully lived as a "male" British military surgeon.
Ashida Kim, believed by many to be Caucasian author and self proclaimed ninja Radford Davis (alternate pen name Christopher Hunter), who wrote numerous books on ninjutsu during the '70s and '80s, noted for refusing to provide details about his teachers, or the lineage of the martial art in which he claims expertise.
Enric Marco, who presided over an association of Spanish survivors of the Nazi camps, when in fact he went to Germany to work in the Nazi war industry.