The Collaborative International Dictionary of English v.0.48:

Noble \No"ble\, a. [Compar. Nobler; superl. Noblest.] [F.
   noble, fr. L. nobilis that can be or is known, well known,
   famous, highborn, noble, fr. noscere to know. See know.]
   1. Possessing eminence, elevation, dignity, etc.; above
      whatever is low, mean, degrading, or dishonorable;
      magnanimous; as, a noble nature or action; a noble heart.
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            Statues, with winding ivy crowned, belong
            To nobler poets for a nobler song.    --Dryden.
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   2. Grand; stately; magnificent; splendid; as, a noble
      edifice.
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   3. Of exalted rank; of or pertaining to the nobility;
      distinguished from the masses by birth, station, or title;
      highborn; as, noble blood; a noble personage.
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   Note: Noble is used in the formation of self-explaining
         compounds; as, noble-born, noble-hearted, noble-minded.
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   Noble gas (Chem.), a gaseous element belonging to group
      VIII of the periodic table of elements, not combining with
      other elements under normal reaction conditions;
      specifically, helium, neon, argon, krypton,
      xenon, or radon; also called inert gas.

   Noble metals (Chem.), silver, gold, and platinum; -- so
      called from their resistance to oxidation by air and to
      dissolution by acids. Copper, mercury, aluminium,
      palladium, rhodium, iridium, and osmium are sometimes
      included.
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   Syn: Honorable; worthy; dignified; elevated; exalted;
        superior; sublime; great; eminent; illustrious;
        renowned; stately; splendid; magnificent; grand;
        magnanimous; generous; liberal; free.
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WordNet (r) 3.0 (2006):

noble gas
    n 1: any of the chemically inert gaseous elements of the helium
         group in the periodic table [syn: noble gas, inert gas,
         argonon]