The Collaborative International Dictionary of English v.0.48:

Ion \I"on\ ([imac]"[o^]n), n. [Gr. 'io`n, neut, of 'iw`n, p. pr.
   of 'ie`nai to go.]
   1. (Elec. Chem.) an atom or goup of atoms (radical) carrying
      an electrical charge. It is contrasted with neutral atoms
      or molecules, and free radicals. Certain compounds, such
      as sodium chloride, are composed of complementary ions in
      the solid (crystalline) as well as in solution. Others,
      notably acids such as hydrogen chloride, may occur as
      neutral molecules in the pure liquid or gas forms, and
      ionize almost completely in dilute aqueous solutions. In
      solutions (as in water) ions are frequently bound
      non-covalently with the molecules of solvent, and in that
      case are said to be solvated. According to the
      electrolytic dissociation theory, the molecules of
      electrolytes are divided into ions by water and other
      solvents. An ion consists of one or more atoms and carries
      one unit charges 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. Aluminum and chlorine are
         elements prepared predominantly by such electrolysis,
         and depends on dissolving compounds in a solvent where
         the element forms ions. Electrolysis is also used in
         refining other metals, such as copper and silver. Cf.
         Anion, Cation.
         [Webster 1913 Suppl.]

   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.
      [Webster 1913 Suppl.]