Qenya
yatta
noun. narrow neck, isthmus, (narrow) neck, isthmus; *(lit.) joining
yat
noun. neck, neck; [ᴱQ.] isthmus
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 word for “narrow neck, isthmus” in The Etymologies of the 1930s (Ety/YAK). Although Christopher Tolkien presented ᴹQ. yatta as if it were a derivative of ᴹ√YAK (LR/400), in their Addenda and Corrigenda to the Etymologies Carl Hostetter and Patrick Wynne indicated it was actually a derivative of a deleted root ᴹ√YATH, that appeared between ᴹ√YAK and ᴹ√YAT “join” (VT46/22). Although ᴹ√YATH was deleted, ᴹQ. yatta was not, and it was possible Tolkien reconceived of it as a derivative of ᴹ√YAT, as suggested by Hostetter and Wynne. As for ᴹ√YAK, Tolkien had a similar word ᴹQ. yat (yaht-) “neck” derived from that root.
Conceptual Development: In the Qenya Lexicon of the 1910s, Tolkien had ᴱQ. yatta “neck, also isthmus” under the early root ᴱ√ẎATA “join”, along with an archaic variant †yat (QL/105). In a list of body parts from the 1920s, Tolkien instead had ᴱQ. yat (yakt-) “neck” (PE14/117).
Neo-Quenya: It is not clear that both yaht- and yatta should coexist. For purposes of Neo-Quenya, I would just use yatta “(narrow) neck, isthmus”, as it has a clearer etymology and ᴹ√YAT has a larger set of derivatives.