From Tolkien Gateway

Undómë was one of the Elves' names for the hour of star-opening at the end of the day: that is, dusk. Undómë was the Quenya name for this time, with the Sindarin equivalent being aduial. There was also a poetic Mannish translation, Evendim (that is, the dim light of evening).[1] This word is the source of Arwen's surname Undómiel, which translates as 'Evenstar'.