The most common Sindarin word for “short”. In The Etymologies of the 1930s it was N. thent “short” derived from ᴹ√STINTĀ of the same meaning where the e was the result of a-affection (Ety/STINTĀ). In 1957 Notes on Names (NN), it was thent < ✶stentā where it was contrasted with thenn “grey-elf” < ✶thindā (PE17/141).
In 1957 Quenya Notes (QN) Tolkien instead had thenn “short” < ✶stenna, but this was part of a (brief?) paradigm where medial nt did not become nn in Sindarin (PE17/185). As such thent could no longer be the basis for Ann-thennath “✱Long-shorts” (LotR/193), name of a form of Elvish verse from The Lord of the Rings, and thus a new etymology was required:
> Since nt &c. no longer are held to yield nn &c., the word for “short” should be ✶stenna √STEN- “cut short, limit, confine, cramp”. Q thenna/senna. S thenn, esten (PE17/185).
However, this word appeared again as thent in the name of another style of Elvish alliterative verse: Minlamad Thent “✱First-echoing Short” in the draft introduction to the tale Narn i Chîn Húrin (WJ/311).
Neo-Sindarin: Since I prefer to assume medial nt did become nn, I would use thent “short” for purposes of Neo-Sindarin.
Q. thenta, senta.