The Collaborative International Dictionary of English v.0.48:

Alkali \Al"ka*li\ (?; 277), n.; pl. Alkalis or Alkalies. [F.
   alcali, ultimately fr. Ar. alqal[imac] ashes of the plant
   saltwort, fr. qalay to roast in a pan, fry.]
   1. Soda ash; caustic soda, caustic potash, etc.
      [1913 Webster]

   2. (Chem.) One of a class of caustic bases, such as soda,
      potash, ammonia, and lithia, whose distinguishing
      peculiarities are solubility in alcohol and water, uniting
      with oils and fats to form soap, neutralizing and forming
      salts with acids, turning to brown several vegetable
      yellows, and changing reddened litmus to blue.
      [1913 Webster]

   3. Soluble mineral matter, other than common salt, contained
      in soils of natural waters. [Western U. S.]
      [Webster 1913 Suppl.]

   Fixed alkalies, potash and soda.

   Vegetable alkalies. Same as Alkaloids.

   Volatile alkali, ammonia, so called in distinction from the
      fixed alkalies.
      [1913 Webster]

WordNet (r) 3.0 (2006):

alkali
    n 1: any of various water-soluble compounds capable of turning
         litmus blue and reacting with an acid to form a salt and
         water; "bases include oxides and hydroxides of metals and
         ammonia" [syn: base, alkali]
    2: a mixture of soluble salts found in arid soils and some
       bodies of water; detrimental to agriculture

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "alkali":
   acid, acidity, agent, alkalinity, alloisomer, anion, antacid, atom,
   base, biochemical, cation, chemical, chemical element,
   chromoisomer, compound, copolymer, dimer, element, heavy chemicals,
   high polymer, homopolymer, hydracid, inorganic chemical, ion,
   isomer, macromolecule, metamer, molecule, monomer, neutralizer,
   nonacid, organic chemical, oxyacid, polymer, pseudoisomer, radical,
   reagent, sulfacid, trimer