The Sindarin word for “eye”, most notably in the name Amon Hen “Hill of the Eye” (LotR/400), derived from the root √KHEN that was the basis for eye-words (PE17/187). Given the words henneth “window” (LotR/674) and Lachend “Flame-eyed” (WJ/384), it is possible that the independent word for “eye” is hend, but note also maecheneb “sharp-eye” which has no double-n (WJ/337).
Conceptual Development: This word dates all the way back to G. hen “eye” in the Gnomish Lexicon of the 1910s (GL/48), cognate to ᴱQ. hend- and so probably similarly derived from primitive ᴱ✶þχe-ndǝ (PE12/21). In the Early Noldorin Grammar of the 1920s, ᴱN. hen(n) “eye” was paired with ᴱQ. sinda (PE13/122), but in Early Noldorin Word-lists from the same period, ᴱN. henn was again cognate with ᴱQ. hen (hend-), both from primitive ᴱ✶ske-ndá. In The Etymologies of the 1930s it was N. {hent, henn >>} hên “eye” from the root ᴹ√KHEN-D-E “eye” (Ety/KHEN-D-E). Thus this word was well established in Tolkien’s mind, but had several variations in its form and derivation.
A word for “sharp-eye” (likely an adjective = “sharp-eyed”) appearing in a discussion of the name Maeglin, a combination of S. maeg “sharp”, (mutated) S. hen “eye” and the adjective suffix S. -eb (WJ/337).