1. [n]  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"
2. [n]  a mixture of soluble salts found in arid soils and some bodies of water; detrimental to agriculture

ALKALI is a 6 letter word that starts with A.

1. \Al"ka*li\ (?; 277), n.; pl. {Alkalis} or {Alkalies}. [F.
   alcali, ultimately fr. Ar. alqal[=i] ashes of the plant
   saltwort, fr. qalay to roast in a pan, fry.]
   1. Soda ash; caustic soda, caustic potash, etc.
   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.
   {Fixed alkalies}, potash and soda.
   {Vegetable alkalies}. Same as {Alkaloids}.
   {Volatile alkali}, ammonia, so called in distinction from the
      fixed alkalies.
   

2. \Al"ka*li\, n.
   Soluble mineral matter, other than common salt, contained in
   soils of natural waters. [Western U. S.]
   

1. Arrhenius base: any chemical that increases the number of free hydroxide ions (OH-) when they're added to a water-based solution. The more ions produced, the stronger the acid.
2. Bronsted or Bronsted-Lowry base: any chemical that acts as a proton acceptor in a chemical reaction.
3. Lewis base: any chemical that donates two electrons to form a covalent bond during a chemical reaction.