- 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.
- 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
