In chemistry, an alkaline is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water, and a solution of a soluble base has a pH greater than 7.0. The adjective alkaline is commonly 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.

The word "alkaline" is derived from Arabic al qalīy (or alkali), meaning the calcined ashes, referring to the original source of alkaline substances. A water-extract of burned plant ashes, called potash and composed mostly of potassium carbonate, was mildly basic. After heating this substance with calcium hydroxide (slaked lime), a far more strongly basic substance known as caustic potash (potassium hydroxide) was produced.

Alkaline chemicals are basic chemicals that produce hydroxide ions (-OH) in solution, and these chemicals have a pH higher than 7.0. Alkaline chemicals like these are mixed with oil or other fats to form soap.

In summary, an alkaline is a basic, ionic salt of an alkali metal or an alkaline earth metal, and it can also 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 used in English as a synonym for basic, especially for bases soluble in water.