chemical compound that. in water solution. reacts as a base by forming hydroxyl ions (OH-). Commonly referred to as metal hydroxides.” “

Merriam-Webster Online Dictionary
alkali (noun)
a soluble salt obtained from the ashes of plants and consisting largely of potassium or sodium carbonate , broadly a substance (as a hydroxide or carbonate of an alkali metal) having marked basic properties - compare base 7a
- alkali metal
a soluble salt or a mixture of soluble salts present in some soils of arid regions in quantity detrimental to agriculture
alkali (Wikipedia)

In chemistry, an alkali (/ˈælkəl/; from Arabic: al-qaly "ashes of the saltwort") is a basic, ionic salt of an alkali metal or alkaline earth metal chemical element. An alkali also can be defined 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 bases soluble in water. 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.

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