Mairon, masc. name "the Admirable" (cf. adj. maira), said to be the original name of Sauron, changed when he was suborned by Melkor, "but he continued to call himself Mairon the Admirable, or Tar-mairon King Excellent, until after the downfall of Númenor" (PE17:183). Since Sauron had joined Melkor before the Elves came to Valinor and developed the Quenya language, we are perhaps to understand that Mairon is a translation by sense of Saurons original Valarin name, though Sauron himself may seem to have used the Elvish form in Middle-earth and on Númenor.
Quenya
mairon
masculine name. Admirable
Mairon
the admirable
maira
admirable, excellent, precious
maira adj. "admirable, excellent, precious"; "splendid, sublime" ("only of great, august or splendid things") (PE17:163, 172). Cf. Mairon.
The original name of Sauron before his corruption by Melkor (PE17/183). Even after his corruption, he still called himself Mairon or Tar-Mairon sometimes, at least until he lost his beautiful form after the fall of Númenor (PE17/183). This name seems to be a masculinized form of the adjective maira “admirable”.