Gnomish
ereth
noun. solitude, oneness, loneliness
tol erethrin
place name. Lonely Isle
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.
An abstract noun based on G. er “one” appearing in the Gnomish Lexicon of the 1910s with the glosses {“oneness, unity; solitude” >>} “solitude, oneness, loneliness” (GL/32).
Neo-Sindarin: Since S. er “one” survives in Tolkien’s later writings and -th remains a common noun suffix, I would retain ᴺS. ereth “solitude, oneness, loneliness” for purposes of Neo-Sindarin.