The uncanny valley is a common unsettling feeling people experience when androids (humanoid robots) and audio/visual simulations closely resemble humans in many respects but are not quite convincingly realistic.
The phenomenon is a consideration in a number of areas of design including robotics, video game art, training simulators and 3-D animation. Depending on the intent, a designer may want to avoid the uncanny valley or exploit it to elicit a particular response.
The uncanny valley is named for the way the viewer's level of comfort drops as a simulation approaches, but does not reach, verisimilitude. Simulations lacking aspects that significantly approach reality don't tend to elicit the response. On the other hand, simulations that simulate reality to a degree that satisfies the viewer don't bring about the effect either. Near-realism and mixes of realism and surrealism most often cause the eerie sensation. The effect is intensified if the simulation is moving.
A plotted graph of viewer response to increased realism illustrates the uncanny valley (Definition continues below the graph):
The feelings diagrammed in the uncanny valley can reach extreme levels like revulsion, exceeding those experienced when viewing a corpse. The uncanny valley is experienced at different levels by different individuals, mostly depending on the familiarity of the subject materials. Designers can bridge the valley with changes like the addition of cartoon-like or "cuter" features.
The uncanny valley phenomenon is usually spoken of in reference to human simulations but also can be brought about by Photoshopped images, dolls, teddy bears and subjects of plastic surgery. The uncanny valley can also be created by discrepancies in voice, movement or appearance.
Masahiro Mori, at that time a robotics professor, wrote about the effect in a 1970 essay, Bukimi no Tani, which translates roughly as valley of eeriness. At that time, humanoid robots had yet to be developed. Mori was intrigued by the uneasy feeling that wax figures had always evoked in him. The English term uncanny valley was first mentioned in a 1978 book by Jasia Reichardt called "Robots: Fact, Fiction, and Prediction."
Popular Science explores the uncanny valley: