To make something darker in color or to reduce the amount of light in a place.
From Old English 'deore' meaning 'dark,' combined with the suffix '-en' which creates verbs meaning 'to become' or 'to make.' The root relates to Proto-Germanic words.
The human eye can detect a room darkening in less than a second—our pupils automatically dilate to catch more light, which is why 'darken' feels like such an immediate change!
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