the lowest level of the ocean that light can reach.
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.
Origin of twilight zone
First recorded in 1905–10
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
any indefinite or transitional condition or area
an area of a city or town, usually surrounding the central business district, where houses have become dilapidated
the lowest level of the ocean to which light can penetrate
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012