twilight zone

noun

1. the lowest level of the ocean that light can reach.
2. an ill-defined area between two distinct conditions, categories, etc., usually comprising certain features of both; an indefinite boundary:

   a twilight zone between fantasy and reality.

twilight zone

noun

1. any indefinite or transitional condition or area
2. an area of a city or town, usually surrounding the central business district, where houses have become dilapidated
3. the lowest level of the ocean to which light can penetrate

Word History and Origins

Origin of twilight zone¹

First recorded in 1905–10

Example Sentences

Overall, the team found that just spending one minute in N1 sleep, the twilight zone, triggered a person to find a hidden rule nearly three times more often than those who were awake but resting.

From Singularity Hub 

Super-LumiNova hands are readable in the sea’s twilight zone.

From Outside Online

The animals in this twilight zone share many characteristics, but among the most important is slow growth, leading to extended adolescence, low fecundity once maturity is attained, and long lifespans.

From Nautilus