twilight; western evening sky
Refers to the lingering light in the western sky after sunset; a somewhat poetic or literary term.
Let's go home before the twilight in the western sky fades.
The western evening glow still lingers beyond the mountains.
Compound of 西 (west) and 明かり (light). The exact historical derivation is uncertain, but the word transparently describes the light seen in the western sky after sunset.