The uncanny valley is a hypothesis that when robots and other facsimiles of humans look and act almost like actual humans, it causes a response of revulsion among human observers.
The "valley" in question is a dip in a proposed graph of the positivity of human reaction as a function of a robot's lifelikeness.
Taken from (and further reading available at) Wikipedia: http://en.wikipedia.org/wiki/Uncanny_valley.
