A sunset, also known as sundown or sunfall, occurred when a star disappeared as it moved beneath the horizon of a planet or moon.
On 2152 Risa, every evening, after sunset, there was a boat which sailed into the Suraya Bay. Then, people would wade to this boat where they were served seafood. (ENT: "Two Days and Two Nights")
In 2364, while suffering from polywater intoxication, Geordi La Forge bemoaned never having seen a rainbow, sunset, or sunrise as Doctor Beverly Crusher administered a hypospray. (TNG: "The Naked Now")
In 2365, after studying five distinct sign languages, Data demonstrated his proficiency in the first of these languages to Captain Jean-Luc Picard by signing, "This is blue. This is a blue ocean. This is a blue ocean at sunset. This is two people walking along the beach by a blue ocean at sunset. This is two happy people in love walking along the beach by a blue ocean at sunset." (TNG: "Loud As A Whisper")