Those reactions that are called aldols derive their name from the early nineteenth-century organic literature. The term was first applied to a self-addition product of acetaldehyde that forms under basic conditions. The product, 3-hydroxybutanal, can form in yields as high as 50% in the presence of sodium hydroxide. This substance eventually became referred to as aldol, because it was both an aldehyde and an alcohol.
The aldol reaction can be defined in the broad sense as a reaction in which a nucleophile-generated alpha to an electron-withdrawing functional group (in the large majority of cases, carbonyl groups are responsible for the α-hydrogen acidity, although they are not required) adds to a carbonyl group (it may be self-condensation as in the example of acetaldehyde, but it may also involve attack on another carbonyl containing species, if it is present). The Knoevenagel reaction as another condensation possessing a mechanism similar to that of the aldol reaction.