twilight zone is a noun meaning "the lowest level of the ocean to which light can reach". It carries 2 recorded senses.
the lowest level of the ocean to which light can reach
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”
Definitions, synonyms and semantic relations for twilight zone come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.