Via Fast Company
The uncanny valley is a term, coined in the seventies by roboticist Masahiro Mori, to describe the point at which a robot becomes human-like enough that it is no longer endearing -- but instead is very creepy.
Popular Mechanics explains:
Visualized as a curve, our sense of familiarity theoretically tracks upward as we encounter increasingly human-like machines. The steep, uncanny drop-off that marks the point of too human-like becomes a valley when you include the subsequent steep rise associated with a real human being, or perfect android. Those robots unlucky enough to topple into the valley are victims of our intimate, hard-wired perception of human biology and social cues.
Now, more than ever, robots can be created that get uncomfortably close to (some would say right in the heart of) the uncanny valley thanks to a new material called "Frubber". Created by Robotics designer David Hanson, the material -- named as a contraction of face + rubber -- is a lightweight polymer that designers can use to create faces for robots that closely emulate natural human facial expressions.