A chemical which neutralises acids.
Alkalis are the hydroxides of the alkali and alkaline earth metals and also ammonia solution. Apart from ammonia, the most common alkalis are derived from sodium (caustic soda), potassium (caustic potash) and calcium (slaked lime). In solution, they have a pH greater than 7 and turn litmus paper blue.
Solutions containing an alkali (alkaline solutions) can dissolve oils and greases on metals and also on skin. They are therefore frequently the active ingredient in metal washing chemicals. Very strong alkaline solutions (caustic solutions) can cause severe skin damage, which looks very much like a burn when it has been cleaned, and is therefore called a chemical burn.
Alkaline means having the properties of an alkali.