The basis word for “hill” in Quenya (MC/222; PE17/90), also referring to “rising ground” in general (PE17/92).
Conceptual Development: The earliest precursor of this word was ᴱQ. †amun (amund-) “hill” in the Qenya Lexicon of the 1910s, a derivative of ᴱ√AM(U) “up(wards)”, though this word was marked as poetic (QL/30); amund- “hill” was also mentioned in the Poetic and Mythological Words of Eldarissa (PME/30). The form ᴱQ. ambo “hill” first appeared in Early Noldorin Word-lists of the 1920s as a cognate of ᴱN. amon (PE13/137, 159), and it (mostly) retained this form thereafter.
ᴹQ. ambo “hill” reappeared in The Etymologies of the 1930s as a derivative of ᴹ√AM “up” (Ety/AM²). In notes on the comparative from early in 1967, Tolkien coined some different roots as the basis for this word, first √MAB “lump, mass” (PE17/90) and then √MBON, the latter producing Q. umbo(n) “hill, lump, clump, mass” (PE17/90-93), his motivation being that he wanted √AMA to have a new meaning “addition, increase, plus” to serve as the basis for the intensive. But in other notes from 1967 he derived Q. ambo from √AM “go up”, though in that note he glossed the word as “mount” (PE17/157). In the Q. Markirya poem from late 1960s, ambo was used for “hill” (MC/222).
Neo-Quenya: For purposes of Neo-Quenya, I would assume Q. ambo was derived from √AM “up”, and would limit its used to hills, ignoring its 1967 “mount” gloss.
ambo noun "hill, rising ground" (Markirya, PE17:92), "mount" (PE17:157), allative pl. ambonnar "upon hills" in Markirya (ruxal' ambonnar "upon crumbling hills") According to VT45:5, ambo was added to the Etymologies as a marginal note.