"light from the sky when the sun is below the horizon at morning and evening," late 14c. (twilighting), a compound of twi- + light (n.) Cognate with Middle Flemish twilicht, Dutch tweelicht (16c.), Middle High German twelicht, German zwielicht. Exact connotation of twi- in this word is unclear, but it appears to refer to "half" light, rather than the fact that twilight occurs twice a day. Compare also Sanskrit samdhya "twilight," literally "a holding together, junction," Middle High German zwischerliecht, literally "tweenlight." Originally and most commonly in English with reference to evening twilight but occasionally used of morning twilight (a sense first attested mid-15c.). Figurative extension recorded from c. 1600.
Twilight zone is from 1901 in a literal sense, a part of the sky lit by twilight; from 1909 in extended senses in references to topics or cases where authority or behavior is unclear. In the 1909 novel "In the Twilight Zone," the reference is to mulatto heritage. "She was in the twilight zone between the races where each might claim her ...." The U.S. TV series of that name is from 1959.