A carbon atom double-bonded to an oxygen, single-bonded to a hydrogen, and single-bonded to another chemical group (such as methane, benzene, another hydrogen, anything). The carbon oxygen double bond part is known as a carbonyl group (-C=O). An example is acetaldehyde, which is a carbonyl group single-bonded to a hydrogen and single-bonded to a methane (a methyl group: -CH3).