Qenya
landa
noun. plain
landa
adjective. wide, wide, [ᴱQ.] broad
lanna
noun. plain
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 noun for “a plain” in the Quenya Verbal System (QVS) of the 1940s derived from ᴹ√LAD “lie flat, be flat” with variants landa and lanna (PE22/126), the latter probably derived from ✱ladna with the voiced stop d becoming a nasal before nasal n. It might simply be the noun form of adjective ᴹQ. landa “wide” from The Etymologies of the 1930s (Ety/LAD).
Neo-Quenya: For purposes of Neo-Quenya, I’d stick to the form landa, which appears in an inflected form landannar “to the plains” early in QVS (PE22/125).