![1girl 2018 absurdres artist_name brown_hair closed_eyes cropped_torso drawn_ears facing_viewer heart highres long_hair mask mouth_mask original realistic scarf shal.e simple_background solo surgical_mask sweater upper_body watermark waving rating:s score:21 post #2975383](/cdn_image/preview/3a/93/3a93e763ee28a751bf914c271401461e.jpg)
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.