A noun in The Lord of the Rings Appendix E glossed “root”, given as an examples of how “nd remained at the end of fully accented monosyllables” (LotR/1115). It was an element in the river-name S. Morthond “Black Root” (LotR/770), so named “because its source was in the dark caverns of the Dead Men” (RC/766). As such this word refers to things that are the root or base of something, not just plant roots.
Conceptual Development: The Etymologies of the 1930s had N. sunn and sonnas as cognates to ᴹQ. sundo “base, root, root-word” under ᴹ√SUD “base, ground” (Ety/SUD; EtyAC/SUD). These Noldorin forms were revised to N. thund/thonn and N. thonnas while the root was revised to ᴹ√STUD (EtyAC/SUD). The Etymologies also had N. dum “root, foundation” derived from {ᴹ√(N)DUM >>} ᴹ√(N)DUB “lay base, foundation, root; found”, but this entry was deleted (EtyAC/NDUB).
Possible Etymology: This words seems to be a counterexample to the general rule that short u was preserved before nasals]]: compare it to S. mund “bull” and N. lhunt “boat” where the u remained unchanged. The Quenya cognate of this word is typically Q. sundo, so a-affection]] cannot be used to explain the shift of u to o. However in one place Tolkien gave the Quenya form as sunda in Tarmasundar “Roots of the Pillar” (UT/166), so perhaps the Sindarin form was derived from a variant primitive form ✱stundā.
Neo-Sindarin: For purposes of Neo-Sindarin, I would assume thond refers only to an ordinary base or root, and more abstract [N.] thonnas refers to things like root-words or a “✱foundation”.
A noun appearing only in its plural form kulbî “roots”, corresponding to the collective-noun kulub “roots, edible vegetables that are roots not fruits” (SD/431). As such, it most likely refers to root vegetables only, rather than other senses of the English word “root”.