noun
westering sun; setting sun; afternoon sun
Refers to the sun shining from the west, especially in the late afternoon or evening. Often carries a nuance of strong, glaring light or heat, and is commonly used when talking about rooms or buildings that face west.
西日がまぶしくて、カーテンを閉めた。
The afternoon sun was so bright that I closed the curtains.
この部屋は西日が強くて、夏はとても暑い。
This room gets strong westering sun, so it's very hot in summer.
夕日 refers specifically to the evening sun or sunset as a scenic phenomenon, while 西日 emphasizes the direction (west) and often the harshness of the afternoon/evening sunlight.
Compound of 西 (west) and 日 (sun). The reading にしび is a straightforward combination of the kun'yomi readings of the two kanji.