Definition of "alkali"

(chemistry) One of a class of caustic bases, such as soda, soda ash, caustic soda, potash, ammonia, and lithia, whose distinguishing characteristics are dissolving in alcohol and water, uniting with oils and fats to form soap, neutralizing and forming salts with acids, turning to brown several vegetable yellows, and changing reddened litmus to blue.  examples

(Western US) Soluble mineral matter, other than common salt, contained in soils of natural waters.  examples