O=C-OH | R (where R is a hydrogen or organic group)
or any molecule containing such a functional group.
Some carboxylic acids include
HCOOH formic acid (found in insect stings, formic refers to ants)
CH3COOH acetic acid (found in vinegar)
CH3CH2COOH propanoic acid
C6H5COOH benzoic acid (known historically as carbolic acid, an early antimicrobial disinfectant?)