any of numerous combinations of atoms that form parts of chemical molecules, that undergo characteristic reactions themselves, and that in many cases influence the reactivity of the remainder of the molecule. In organic chemistry the concept of functional groups is useful as a basis for classification of large numbers of compounds according to their reactions.

Some of the common functional groups are hydroxyl, present in alcohols and phenols; carboxyl, present in carboxylic acids; carbonyl, present in aldehydes, ketones, and quinones; and nitro, present in certain organic nitrogen compounds.