carboxylPronunciation: /kɑːˈbɒksʌɪl, -sɪl/ (also carboxy)Definition of carboxyl noun [as modifier] Chemistry of or denoting the acid radical COOH, present in most organic acids:the carboxyl groupOrigin: mid 19th century: from carbo- + ox- 'oxygen' + -ylcarboxyl in other Oxford dictionaries Definition of carboxyl in the US English dictionary