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Webster 1913 Edition

Noun.

Twilight; gloaming.

[Prov. Eng. & Scot.]

Wright.

ppr.

Depriving of light; obscuring; making black or less white or clear; clouding.

darkening

darkening ‎(plural darkenings)

The act of something becoming darker.
- 1978, Gerald Leonard Howett, Kenneth Low Kelly, E. Thomas Pierce, Emergency vehicle warning lights: state of the art
  NEGATIVE FLASHES — Brief darkenings of a light that is otherwise steadily on.