In chemistry, an alkali ( from Arabic: al-qaly القلي, القالي ) is a basic, ionic salt of an alkali metal or alkaline earth metal chemical element.
Some authors also define an alkali as a base that dissolves in water.
A solution of a soluble base has a pH greater than 7.0. The adjective alkaline is commonly, and alkalescent less often, used in English as a synonym for basic, especially for soluble bases.
This broad use of the term is likely to have come about because alkalis were the first bases known to obey the Arrhenius definition of a base, and they are still among the most common bases.
source: adapted from http://en.wikipedia.org/w/index.php?title=Alkali&oldid=608595881