The Collaborative International Dictionary of English v.0.48:

Salt \Salt\, n. [AS. sealt; akin to OS. & OFries. salt, D. zout,
   G. salz, Icel., Sw., & Dan. salt, L. sal, Gr. ?, Russ. sole,
   Ir. & Gael. salann, W. halen, of unknown origin. Cf. Sal,
   Salad, Salary, Saline, Sauce, Sausage.]
   1. The chloride of sodium, a substance used for seasoning
      food, for the preservation of meat, etc. It is found
      native in the earth, and is also produced, by evaporation
      and crystallization, from sea water and other water
      impregnated with saline particles.
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   2. Hence, flavor; taste; savor; smack; seasoning.
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            Though we are justices and doctors and churchmen . .
            . we have some salt of our youth in us. --Shak.
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   3. Hence, also, piquancy; wit; sense; as, Attic salt.
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   4. A dish for salt at table; a saltcellar.
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            I out and bought some things; among others, a dozen
            of silver salts.                      --Pepys.
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   5. A sailor; -- usually qualified by old. [Colloq.]
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            Around the door are generally to be seen, laughing
            and gossiping, clusters of old salts. --Hawthorne.
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   6. (Chem.) The neutral compound formed by the union of an
      acid and a base; thus, sulphuric acid and iron form the
      salt sulphate of iron or green vitriol.
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   Note: Except in case of ammonium salts, accurately speaking,
         it is the acid radical which unites with the base or
         basic radical, with the elimination of hydrogen, of
         water, or of analogous compounds as side products. In
         the case of diacid and triacid bases, and of dibasic
         and tribasic acids, the mutual neutralization may vary
         in degree, producing respectively basic, neutral, or
         acid salts. See Phrases below.
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   7. Fig.: That which preserves from corruption or error; that
      which purifies; a corrective; an antiseptic; also, an
      allowance or deduction; as, his statements must be taken
      with a grain of salt.
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            Ye are the salt of the earth.         --Matt. v. 13.
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   8. pl. Any mineral salt used as an aperient or cathartic,
      especially Epsom salts, Rochelle salt, or Glauber's salt.
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   9. pl. Marshes flooded by the tide. [Prov. Eng.]
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   Above the salt, Below the salt, phrases which have
      survived the old custom, in the houses of people of rank,
      of placing a large saltcellar near the middle of a long
      table, the places above which were assigned to the guests
      of distinction, and those below to dependents, inferiors,
      and poor relations. See Saltfoot.
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            His fashion is not to take knowledge of him that is
            beneath him in clothes. He never drinks below the
            salt.                                 --B. Jonson.
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   Acid salt (Chem.)
      (a) A salt derived from an acid which has several
          replaceable hydrogen atoms which are only partially
          exchanged for metallic atoms or basic radicals; as,
          acid potassium sulphate is an acid salt.
      (b) A salt, whatever its constitution, which merely gives
          an acid reaction; thus, copper sulphate, which is
          composed of a strong acid united with a weak base, is
          an acid salt in this sense, though theoretically it is
          a neutral salt.

   Alkaline salt (Chem.), a salt which gives an alkaline
      reaction, as sodium carbonate.

   Amphid salt (Old Chem.), a salt of the oxy type, formerly
      regarded as composed of two oxides, an acid and a basic
      oxide. [Obsolescent]

   Basic salt (Chem.)
      (a) A salt which contains more of the basic constituent
          than is required to neutralize the acid.
      (b) An alkaline salt.

   Binary salt (Chem.), a salt of the oxy type conveniently
      regarded as composed of two ingredients (analogously to a
      haloid salt), viz., a metal and an acid radical.

   Double salt (Chem.), a salt regarded as formed by the union
      of two distinct salts, as common alum, potassium aluminium
      sulphate. See under Double.

   Epsom salts. See in the Vocabulary.

   Essential salt (Old Chem.), a salt obtained by
      crystallizing plant juices.

   Ethereal salt. (Chem.) See under Ethereal.

   Glauber's salt or Glauber's salts. See in Vocabulary.

   Haloid salt (Chem.), a simple salt of a halogen acid, as
      sodium chloride.

   Microcosmic salt. (Chem.). See under Microcosmic.

   Neutral salt. (Chem.)
      (a) A salt in which the acid and base (in theory)
          neutralize each other.
      (b) A salt which gives a neutral reaction.

   Oxy salt (Chem.), a salt derived from an oxygen acid.

   Per salt (Old Chem.), a salt supposed to be derived from a
      peroxide base or analogous compound. [Obs.]

   Permanent salt, a salt which undergoes no change on
      exposure to the air.

   Proto salt (Chem.), a salt derived from a protoxide base or
      analogous compound.

   Rochelle salt. See under Rochelle.

   Salt of amber (Old Chem.), succinic acid.

   Salt of colcothar (Old Chem.), green vitriol, or sulphate
      of iron.

   Salt of hartshorn. (Old Chem.)
      (a) Sal ammoniac, or ammonium chloride.
      (b) Ammonium carbonate. Cf. Spirit of hartshorn, under
          Hartshorn.

   Salt of lemons. (Chem.) See Salt of sorrel, below.

   Salt of Saturn (Old Chem.), sugar of lead; lead acetate; --
      the alchemical name of lead being Saturn.

   Salt of Seignette. Same as Rochelle salt.

   Salt of soda (Old Chem.), sodium carbonate.

   Salt of sorrel (Old Chem.), acid potassium oxalate, or
      potassium quadroxalate, used as a solvent for ink stains;
      -- so called because found in the sorrel, or Oxalis. Also
      sometimes inaccurately called salt of lemon.

   Salt of tartar (Old Chem.), potassium carbonate; -- so
      called because formerly made by heating cream of tartar,
      or potassium tartrate. [Obs.]

   Salt of Venus (Old Chem.), blue vitriol; copper sulphate;
      -- the alchemical name of copper being Venus.

   Salt of wisdom. See Alembroth.

   Sedative salt (Old Med. Chem.), boric acid.

   Sesqui salt (Chem.), a salt derived from a sesquioxide base
      or analogous compound.

   Spirit of salt. (Chem.) See under Spirit.

   Sulpho salt (Chem.), a salt analogous to an oxy salt, but
      containing sulphur in place of oxygen.
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The Collaborative International Dictionary of English v.0.48:

Epsom salts \Ep"som salts`\ or Epsom salt \Ep"som salt`\ (Med.)
   Sulphate of magnesia having cathartic qualities; --
   originally prepared by boiling down the mineral waters at
   Epsom, England, -- whence the name; afterwards prepared from
   sea water; but now from certain minerals, as from siliceous
   hydrate of magnesia.
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WordNet (r) 3.0 (2006):

Epsom salts
    n 1: hydrated magnesium sulfate that is taken orally to treat
         heartburn and constipation and injected to prevent seizures
         [syn: Epsom salts, bitter salts]
    2: (used with a singular noun) hydrated magnesium sulfate used
       as a laxative