1. The descent of the sun below the horizon; also, the time when the sun sets; evening. Also used figuratively. 'T is the sunset of life gives me mystical lore. (Campbell)
2. Hence, the region where the sun sets; the west.
(Science: zoology) Sunset shell, a west Indian marine bivalve (tellina radiata) having a smooth shell marked with radiating bands of varied colours resembling those seen at sunset or before sunrise.
Synonym: rising sun.