The twilight zone is a realm of imagination that seems impossible but is difficult to falsify. It typically challenges an ordinary view of reality.
The concept originated in 1949 and became the basis for an immensely successful television series called The Twilight Zone. One of its most famous episodes concerns the possible role that conformity plays in shaping perceptions of human beauty.
Another famous Twilight Zone episode, in which William Shatner starred, concerned a perception of a monster standing on the wing of a commercial airplane in mid-flight. Others could not falsify the claims by Shatner about what he was seeing.
The Merriam-Webster definition for the term is inadequate: "a word of fantasy, illusion or unreality."