The terms "base" and "alkali" are often used interchangeably, particularly outside the context of chemistry and chemical engineering.
There are various, more specific definitions for the concept of an alkali. Alkalis are usually defined as a subset of the bases. One of two subsets is commonly chosen.
* A basic salt of an alkali metal or alkaline earth metal (this includes Mg(OH) (magnesium hydroxide) but excludes NH (ammonia)).
* Any base that is soluble in water and forms hydroxide ions or the solution of a base in water. (This includes both Mg(OH) and NH, which forms NHOH.)
The second subset of bases is also called an "Arrhenius base".