Definition of 'alkali'

any base or hydroxide, as soda, potash, etc. that is soluble in water and gives a high concentration of hydroxyl ions in solution; specif., any of the hydroxides and carbonates of the alkali metals

any soluble substance, as a mineral salt or mixture of salts, that can neutralize acids, has a pH greater than 7.0, and turns litmus blue: strong alkalies are caustic