Aeluin, sometimes referred to as Tarn Aeluin, was a clear blue mountain lake in the highlands of Dorthonion, said to have been hallowed by Melian the Maia.
There Aegnor saw the reflection of Andreth and fell in love with her.
The outlaws of Barahir hid from Morgoth's forces after the Dagor Bragollach.
Aeluin means "Blue Lake" in Sindarin (from ael = "lake, pool" and luin = "blue"). Tarn is an archaic English word meaning "mountain lake".