A stereotype is a mistaken idea or belief many people have about a thing or group that is based upon how they look on the outside, which may be untrue or only partly true. Stereotyping people is a type of prejudice because what is on the outside is a small part of who a person is. Like other untrue opinions, stereotypes might be used as reasons to discriminate against another person, or sometimes for a humorous effect in fiction.
The term was invented in the late 18th century for a method of printing. In the mid-19th century it meant the faithful reproduction of everything which could be printed, whether words or pictures. It made the printing of cheap editions possible, and was used in printing newspapers. The term got its psychological meaning in the 20th century.