The lake where Barahir and his men made their lair in their fight against the forces of Morgoth (S/162). This name generally appears as “Tarn Aeluin”, but “tarn” is simply an obscure English word for a mountain lake. This name seems to be a compound of ael “lake, pool” and the adjective luin “blue”, since it was described as “clear and blue by day” (S/162).