The Fifth Dimension, also known as The Twilight Zone, is the titular setting in the Twilight Zone franchise.
As defined by The Narrator:
"There is a fifth dimension beyond that which is known to man. It is a dimension as vast as space and 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 fears and the summit of his knowledge. This is the dimension of imagination. It is an area we call the Twilight Zone."