greyline

English Wikipedia has articles on:

terminator (solar) and terminator (solar) § Grey-line radio propagation

Wikipedia Wikipedia

• grayline

From grey +‎ line.

greyline (uncountable)

1. (strictly, technical) A transition line, coinciding with a twilight zone (terminator), where a change in the propagation of radio waves is observed.
2. (loosely) Synonym of twilight zone (“terminator”).

• grey area