A noun mentioned in several places as a translation of “neck” as in Tarlang’s Neck (LotR/790), in Sindarin properly S. Achad Tarlang (PE17/92, 146; RC/537). It was a derivative of √AKAS “neck, ridge” (PE17/146). Somewhat curiously, S. lang “neck” was also an element in the name S. Tarlang. As Tolkien explained it:

It is not certain whether this was named after some ancient man with the Sindarin name Tarlang (“stiff-neck” sc. “proud”) or was due to the obsolescence of Tarlang “the stiff, tough, passage” to which S achad had been prefixed in explanation, so that Achad Tarlang “the crossing called Tarlang” was interpreted as “Tarlang’s Neck”. Achad is another word for “neck”, referring properly only to the vertebrae (the bony part of the neck not including throat). √AKAS: Q akas (later akse) pl. aksi, S ach, geographically achad (< aks). This was also applied geographically to rock ridges. The Neck was not the name of the passage but of the lower, narrower ridge (crossed by the road) between the main mountains, and the mountainous region at southern end of the spur (PE17/92).

Hence S. lang refers to the entire throat, the passage from the mouth to the lungs/stomach, whereas S. ach was the vertebrae and upper spine, and achad was for geographic features resembling the spine, hence rock ridges.