In his essay The Uncanny, Freud sets out to explore and define exactly what is is that what we identify as instances of the uncanny have in common.
He begins by exploring the origin of the word. The German for uncanny is unheimlich, which is the opposite of heimlich, which means homely, or familiar. Therefore, he argues, it is tempting to conclude “that what is ‘uncanny’ is frightening precisely because it is not known and familiar.” However, although some new things might be frightening, not all are. So he determines that “something has to be added to what is novel and unfamiliar in order to make it uncanny.”