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.
      [1913 Webster]

            Statues, with winding ivy crowned, belong
            To nobler poets for a nobler song.    --Dryden.
      [1913 Webster]

   2. Grand; stately; magnificent; splendid; as, a noble
      edifice.
      [1913 Webster]

   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.
      [1913 Webster]

   Note: Noble is used in the formation of self-explaining
         compounds; as, noble-born, noble-hearted, noble-minded.
         [1913 Webster]

   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.
      [1913 Webster]

   Syn: Honorable; worthy; dignified; elevated; exalted;
        superior; sublime; great; eminent; illustrious;
        renowned; stately; splendid; magnificent; grand;
        magnanimous; generous; liberal; free.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Radon \Ra"don\ (r[=a]"d[o^]n), n. (Chem.)
   An intensely radioactive gaseous element produced by the
   radioactive decay of radium-226, which is the main isotope of
   radium found in pitchblende. Chemically it is an inert noble
   gas. Its atomic symbol is Rn. It has an atomic number of 86.
   The radon isotope produced by decay of radium has an atomic
   weight of 222.017, and this isotope decays by alpha emission
   with a half-life of 3.82 days. Numerous other isotopes have
   been observed, all radioactive and all having half-lives
   shorter than that of radon-222. Radon was discovered by M.
   and Mme. Curie, of Paris, in their studies of the radioactive
   substances in pitchblende. Radon was originally called
   radium emanation or exradio.
   [PJC]

WordNet (r) 3.0 (2006):

radon
    n 1: a radioactive gaseous element formed by the disintegration
         of radium; the heaviest of the inert gasses; occurs
         naturally (especially in areas over granite) and is
         considered a hazard to health [syn: radon, Rn, atomic
         number 86]

The Elements (07Nov00):

radon
Symbol: Rn
Atomic number: 86
Atomic weight: (222)
Colorless radioactive gaseous element, belongs to the noble gases. Of
the
twenty known isotopes, the most stable is Rn-222 with a half-life of 3.8
days.
Formed by the radioactive decay of Radium-226. Radon itself decays
into
polonium. Used in radiotherapy. As a noble gas, it is effectively
inert,
though radon fluoride has been synthesized. First isolated in 1908 by
Ramsey
and Gray.