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A disguise can be anything which conceals or changes a person's physical appearance, including a wig, glasses, makeup, costume or other ways. Camouflage is a type of disguise for people, animals and objects. Hats, glasses, change in hair style or wigs, plastic surgery, and make-up are also used.
Disguises can be used by, but not limited to, criminals and by secret agents seeking to avoid identification. A person working for an agency trying to get information might go 'undercover' to get information without being recognised by the public; a well-known person or celebrity may go 'incognito' in order to avoid unwelcome press attention. In comic books and films disguises are used by superheroes and in science fiction by aliens. Dressing up in costumes is a Halloween tradition.
In fiction 
In comic book and superhero stories, disguises are used to hide secret identities and keep special powers secret from ordinary people. For example, Superman passes himself off as Clark Kent, and Spider-Man disguises himself in a costume so that he cannot be recognized as Peter Parker.
In epic poetry, Odysseus uses the disguise of a beggar to test his family's and servants' loyalty upon his return from a 20 year voyage.
Disguise is sometimes used in criminal activity and in spying, and is a common trope in detective fiction and in spy stories. Sherlock Holmes often disguised himself as somebody else to avoid being recognized.
Also, the Clay Camel, one of Mandrake the Magician's foes is considered the Master of the Disguise, because he's able to mimic anyone and change his appearance in seconds.