in organic chemistry, a divalent chemical unit consisting of a carbon (C) and an oxygen (O) atom connected by a double bond. The group is a constituent of carboxylic acids, esters, anhydrides, acyl halides, amides, and quinones, and it is the characteristic functional group (reactive group) of aldehydes (aldehyde) and ketones (ketone). Carboxylic acids (and their derivatives), aldehydes, ketones, and quinones are also known collectively as carbonyl compounds.

Because of a difference in the electron affinities of the carbon and oxygen atoms, the electron pairs that constitute the double bond are held closer to the oxygen atom than to the carbon atom; the electron-rich oxygen atom acquires a negative charge and the electron-deficient carbon atom a positive charge. Thus, molecules containing the carbonyl group are polar. Compounds containing a carbonyl group have higher melting and boiling points than hydrocarbons containing the same number of carbon atoms and are more soluble in polar solvents such as water. The carbonyl group can enter into a variety of chemical reactions; nucleophilic reagents (electron-rich reagents) are attracted to the carbon atom, whereas electrophilic reagents (electron-seeking reagents) are attracted to the oxygen atom.

Aldehydes and ketones contain carbonyl groups attached to alkyl or aryl groups and a hydrogen atom or both. These groups have little effect on the electron distribution in the carbonyl group; thus, the properties of aldehydes and ketones are determined by the behaviour of the carbonyl group. In carboxylic acids and their derivatives, the carbonyl group is attached to one of the halogen atoms or to groups containing atoms such as oxygen, nitrogen, or sulfur. These atoms do affect the carbonyl group, forming a new functional group with distinctive properties.