Twilight zone definition

(1) The ambiguous region between two categories or states or conditions (usually containing some features of both)

"But there is still a twilight zone, the tantalizing occurrences that are probably noise but might possibly be a signal"

"In the twilight zone between humor and vulgarity"

"In that no man's land between negotiation and aggression"

(2) The lowest level of the ocean to which light can reach