The word "alkali" is formed from two Arabic words, "al," meaning "the," and "kali," the name of a plant from the ashes of which soda was obtained.

Chemically, an alkali is one of a class of caustic bases, the term "base" being given to the principal element of a compound. The chief characteristics of an alkali are comparatively well known: it unites with oils and fats to form soaps; neutralizes acids and forms with them fresh compounds known as salts, which possess properties differing from those of either constituent; turns red litmus paper blue; and is soluble in water.

Those alkalies which are obtained in a solid state, viz., soda and potash, are termed "fixed," while ammonia, by reason of its gaseous nature, is called a "volatile" alkali.

An Alkaloid is an alkaline principle found in the tissues of plants or animals, or prepared synthetically in the laboratory; more soluble in alcohol than in water, and having a definite composition as regards the proportions of its chemical elements. This composition is different from that of an alkali. An alkaloid also unites with acids to form salts, and these salts have the same physiological and therapeutic actions as the alkaloid and are nearly all soluble in water.