pentanoic acidPronunciation: /ˌpɛntəˈnəʊɪk/Definition of pentanoic acid noun [mass noun] Chemistry a colourless liquid fatty acid present in various plant oils, used in making perfumes. Chemical formula: CH3(CH2)3COOH Origin: 1920s: pentanoic from pentanepentanoic acid in other Oxford dictionaries Definition of pentanoic acid in the US English dictionary