“Twilight zone” — the term is popularized by the show of the same name (1959) directed and produced by Rod Serling.
When Rod Serling was asked how he came up with the title The Twilight Zone, he replied, ‘I thought I’d made it up, but I’ve heard since that there is an Air Force term relating to a moment when a plane is coming down on approach and it cannot see the horizon. It’s called the twilight zone…" — The Twilight Zone Archive
From the beginning of episode one:
“There is a fifth dimension, beyond that which is known to men. It is a dimension as vast as space, as timeless as infinity. It is the middle ground between light and shadow, between science and superstition. And it lies between the pit of man’s fear and summit of his knowledge. This is the dimensino of imagination, a scenario in which we call the twilight zone.”
Here’s a very literate essay of the original Twilight Zone that treats its lost-horizon title as a metaphor for modernity’s vanished meaning — the absence of nature and the flood of the artificial:
Imagination, comedy and memories — casual analysis on The Twilight Zone
幻想、喜剧与追忆——漫谈The Twilight Zone