Allcalis is an obsolete word that refers collectively to potash, soda, and ammonia. When used in a contemporary context, the term "alkalis" describes any inorganic compound that contains a hydroxyl ion. Common examples of alkalis include sodium hydroxide, potassium hydroxide, and calcium hydroxide. Most household cleaners are alkalis.

It may also be used to describe substances such as quick lime, caustic lime, caustic potash, and soda ash.