Uncanny

All the research keywords lead to the last one left - uncanny, first introduced in 1906 by German psychiatrist Ernst Jentsch. In his essay On the Psychology of the Uncanny, he describes it as 

a product of intellectual uncertainty; so that the uncanny would always, as it were, be something one does not know one’s way about in. The better orientated in his environment a person is, the less readily will he get the impression of something uncanny in regard to the objects and events in it

and, further

In telling a story one of the most successful devices for easily creating uncanny effects is to leave the reader in uncertainty whether a particular figure in the story is a human being or an automaton and to do it in such a way that his attention is not focused directly upon his uncertainty, so that he may not be led to go into the matter and clear it up immediately.

Wikipedia, definition of uncanny

Just like ambience, this word describes an elusive concept that is difficult to define precisely. It's a fleeting feeling, a moment and yet, the word uncanny seems to appear commonly in most books on film theory I have read. It is and has always been the most exciting element of design for me and is something I have been and will continue trying to achieve.