The Collaborative International Dictionary of English v.0.48:

Manganese \Man`ga*nese"\, n. [F. mangan[`e]se, It. manganese,
   sasso magnesio; prob. corrupted from L. magnes, because of
   its resemblance to the magnet. See Magnet, and cf.
   Magnesia.] (Chem.)
   An element obtained by reduction of its oxide, as a hard,
   grayish white metal, fusible with difficulty (melting point
   1244[deg] C), but easily oxidized. Its ores occur abundantly
   in nature as the minerals pyrolusite, manganite, etc. Symbol
   Mn. Atomic number 25; Atomic weight 54.938 [C=12.011].
   [1913 Webster +PJC]

   Note: An alloy of manganese with iron (called ferromanganese)
         is used to increase the density and hardness of steel.
         [1913 Webster]

   Black oxide of manganese, Manganese dioxide or Manganese
   peroxide, or Black manganese (Chem.), a heavy black powder
      MnO2, occurring native as the mineral pyrolusite, and
      valuable as a strong oxidizer; -- called also familiarly
      manganese. It colors glass violet, and is used as a
      decolorizer to remove the green tint of impure glass.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mn \Mn\, n. (Chem.)
   The chemical symbol for manganese.
   [PJC]

WordNet (r) 3.0 (2006):

manganese
    n 1: a hard brittle grey polyvalent metallic element that
         resembles iron but is not magnetic; used in making steel;
         occurs in many minerals [syn: manganese, Mn, atomic
         number 25]

The Elements (07Nov00):

manganese
Symbol: Mn
Atomic number: 25
Atomic weight: 54.938
Grey brittle metallic transition element. Rather electropositive,
combines
with some non-metals when heated. Discovered in 1774 by Scheele.