Source : Webster's Revised Unabridged Dictionary (1913)

Ion \I"on\, n.
   1. One of the electrified particles into which, according to
      the electrolytic dissociation theory, the molecules of
      electrolytes are divided by water and other solvents. An
      ion consists of one or more atoms and carries a unit
      charge of electricity, 3.4 x 10^{-10} electrostatic units,
      or a multiple of this. Those which are positively
      electrified (hydrogen and the metals) are called
      {cations}; negative ions (hydroxyl and acidic atoms or
      groups) are called {anions}.

   Note: Thus, hydrochloric acid ({HCl}) dissociates, in aqueous
         solution, into the hydrogen ion, H^{+}, and the
         chlorine ion, Cl^{-}; ferric nitrate, {Fe(NO3)3},
         yields the ferric ion, Fe^{+++}, and nitrate ions,
         NO3^{-}, NO3^{-}, NO3^{-}. When a solution containing
         ions is made part of an electric circuit, the cations
         move toward the cathode, the anions toward the anode.
         This movement is called migration, and the velocity of
         it differs for different kinds of ions. If the
         electromotive force is sufficient, electrolysis ensues:
         cations give up their charge at the cathode and
         separate in metallic form or decompose water, forming
         hydrogen and alkali; similarly, at the anode the
         element of the anion separates, or the metal of the
         anode is dissolved, or decomposition occurs.

   2. One of the small electrified particles into which the
      molecules of a gas are broken up under the action of the
      electric current, of ultraviolet and certain other rays,
      and of high temperatures. To the properties and behavior
      of ions the phenomena of the electric discharge through
      rarefied gases and many other important effects are
      ascribed. At low pressures the negative ions appear to be
      electrons; the positive ions, atoms minus an electron. At
      ordinary pressures each ion seems to include also a number
      of attached molecules. Ions may be formed in a gas in
      various ways.