The Collaborative International Dictionary of English v.0.48:

Potassium \Po*tas"si*um\, n. [NL. See Potassa, Potash.]
   (Chem.)
   An Alkali element, occurring abundantly but always combined,
   as in the chloride, sulphate, carbonate, or silicate, in the
   minerals sylvite, kainite, orthoclase, muscovite, etc. Atomic
   weight 39.0. Symbol K (Kalium).
   [1913 Webster]

   Note: It is reduced from the carbonate as a soft white metal,
         lighter than water, which oxidizes with the greatest
         readiness, and, to be preserved, must be kept under
         liquid hydrocarbons, as naphtha or kerosene. Its
         compounds are very important, being used in glass
         making, soap making, in fertilizers, and in many drugs
         and chemicals.
         [1913 Webster]

   Potassium permanganate, the salt KMnO4, crystallizing in
      dark red prisms having a greenish surface color, and
      dissolving in water with a beautiful purple red color; --
      used as an oxidizer and disinfectant. The name chameleon
      mineral is applied to this salt and also to potassium
      manganate.

   Potassium bitartrate. See Cream of tartar, under Cream.
      [1913 Webster]

WordNet (r) 3.0 (2006):

potassium
    n 1: a light soft silver-white metallic element of the alkali
         metal group; oxidizes rapidly in air and reacts violently
         with water; is abundant in nature in combined forms
         occurring in sea water and in carnallite and kainite and
         sylvite [syn: potassium, K, atomic number 19]

The Elements (07Nov00):

potassium
Symbol: K
Atomic number: 19
Atomic weight: 39.0983
Soft silvery metallic element belonging to group 1 of the periodic table
(alkali metals). Occurs naturally in seawater and a many minerals.
Highly
reactive, chemically, it resembles sodium in its behavior and compounds.
Discovered by Sir Humphry Davy in 1807.