The uncanny valley is a term coined by a Japanese robotocist in the 1970s to refer to the strong reaction of revulsion people are believed to have to robots that look uncannily realistic, but fall just short of being convincing. However, the uncanny valley theory also suggests that if robots become human enough, people's emotional response to them will turn positive again. The term, therefore, refers to the dip in people's feelings toward robots based on how human they appear to be.
This term is used in many contexts, but is becoming increasingly common in the context of sex robots and ultra-lifelike sex dolls.