A noun for “twilight” developed from primitive ᴹ✶dōmi (Ety/DOMO), where the long [[ilk|[ō] became [ū] before [m]]].
Doriathrin
muil
noun. twilight, shadow, vagueness
dûm
noun. twilight
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
A Doriathrin noun meaning “twilight, shadow, vagueness” derived from root ᴹ√MUY (Ety/MUY). Its Quenya cognate ᴹQ. muile suggests a primitive form of ✱✶muı̯lē [mujlē].
Conceptual Development: Tolkien seems to have coined this word to explain Umboth Muilin “Twilight Meres”, a remnant of earlier G. Umboth-muilin “Pools of Twilight”. The meaning of the Ilkorin elements is reversed from the earlier name: in the Gnomish name muilin “pools” was the plural form of the word G. muil, glossed “tarn” in the Gnomish Lexicon (GL/58) which is an older English word for “(mountain) lake”. In later writings, Tolkien replaced this name with S. Aelin-uial “Meres of Twilight”.