A carboxyl group is a functional group consisting of a carbon atom double-bonded to an oxygen atom and single-bonded to another oxygen which is in turn bonded to a hydrogen atom (-COOH).
Related terms
Carboxylic Acid: Organic acids characterized by the presence of at least one carboxyl group.
Ester: A compound made by replacing the hydrogen of an acid by an alkyl or other organic group.
Amide: An organic compound that contains a carbonyl (C=O) group joined to nitrogen.