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From 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.]
.

From The Collaborative International Dictionary of English v.0.48:

Symbol \Sym"bol\ (s[i^]m"b[o^]l), n. [L. symbolus, symbolum, Gr.
   sy`mbolon a sign by which one knows or infers a thing, from
   symba`llein to throw or put together, to compare; sy`n with +
   ba`llein to throw: cf. F. symbole. Cf. Emblem, Parable.]
   1. A visible sign or representation of an idea; anything
      which suggests an idea or quality, or another thing, as by
      resemblance or by convention; an emblem; a representation;
      a type; a figure; as, the lion is the symbol of courage;
      the lamb is the symbol of meekness or patience.
      [1913 Webster]

            A symbol is a sign included in the idea which it
            represents, e. g., an actual part chosen to
            represent the whole, or a lower form or species used
            as the representative of a higher in the same kind.
                                                  --Coleridge.
      [1913 Webster]

   2. (Math.) Any character used to represent a quantity, an
      operation, a relation, or an abbreviation.
      [1913 Webster]

   Note: In crystallography, the symbol of a plane is the
         numerical expression which defines its position
         relatively to the assumed axes.
         [1913 Webster]

   3. (Theol.) An abstract or compendium of faith or doctrine; a
      creed, or a summary of the articles of religion.
      [1913 Webster]

   4. [Gr. ? contributions.] That which is thrown into a common
      fund; hence, an appointed or accustomed duty. [Obs.]
      [1913 Webster]

            They do their work in the days of peace . . . and
            come to pay their symbol in a war or in a plague.
                                                  --Jer. Taylor.
      [1913 Webster]

   5. Share; allotment. [Obs.]
      [1913 Webster]

            The persons who are to be judged . . . shall all
            appear to receive their symbol.       --Jer. Taylor.
      [1913 Webster]

   6. (Chem.) An abbreviation standing for the name of an
      element and consisting of the initial letter of the Latin
      or New Latin name, or sometimes of the initial letter with
      a following one; as, C for carbon, Na for sodium
      (Natrium), Fe for iron (Ferrum), Sn for tin (Stannum),
      Sb for antimony (Stibium), etc. See the list of names
      and symbols under Element.
      [1913 Webster]

   Note: In pure and organic chemistry there are symbols not
         only for the elements, but also for their grouping in
         formulas, radicals, or residues, as evidenced by their
         composition, reactions, synthesis, etc. See the diagram
         of Benzene nucleus, under Benzene.
         [1913 Webster]

   Syn: Emblem; figure; type. See Emblem.
        [1913 Webster]
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