A word for the seeing stones of Númenor, a combination of palan “far” and some form of the root TIR “see” (Let/427), hence: “far-seer” or “far-gazer” (LotR Index; PE17/25, 86). This word is unusual in that it has a long vowel in its final syllable, something that does not normally occur in Quenya words. The ancient form of this word was palantīrā̆ (Let/427), and the reduction of short vowels in long compounds was not unusual in Quenya, for example: Valinor as a shorter form of Valinórë. However, after such reductions long vowels in final syllables tended to shorten, so the expected form would be palantir, not palantír. In one place Tolkien described this word as a “a Numenorean formation” (PE17/86), perhaps as a way of explaining the unusual retention of a long vowel in its final syllable.

Because of this long vowel, the proper pronunciation of this word is in dispute. The usual rules for Quenya stress would put the stress on the second syllable: paLANtír. The speech coaches for Peter Jackson’s Lord of the Rings movies instructed the actors to pronounce this word like English “lantern” to help them remember this stress pattern. However in private notes Tolkien wrote pálan-tìr (PE17/86), indicating primary stress on the first syllable and secondary stress on the last: PAlanTÍR. Notes from the Quendi and Eldar essay of 1959-60 seem to indicate the stress shift to the middle syllable occurs only for older compounds:

These compounds being old [my emphasis] were accented as unitary words and the main stress came on the syllable preceding -quen: kirya:quen [kirYAquen], kirya:queni [the implication being the stress remains as a pair of words in later compounds] (WJ/407 note #3).

Hat-tip to Raccoon and Vyacheslav Stepanov for this discovery and pointing it out to me. Assuming this reasoning is correct, the long vowel in the final syllable of palantír may simply be to emphasize the unusual stress pattern; compare also María which also has an abnormal long vowel enforcing a stress pattern that would be atypical for Quenya.

Conceptual Development: This word appeared in The Etymologies of the 1930s where it was probably a later addition. In its earliest appearances, both in The Etymologies and the drafts of The Lord of the Rings, it was Palantir with a short i (Ety/PAL, TIR; WR/76).