acetonitrilePronunciation: /əˌsiːtə(ʊ)ˈnʌɪtrʌɪl, ˌasɪtəʊ-/Definition of acetonitrile noun [mass noun] Chemistry an odoriferous toxic liquid, used as a solvent in high-performance liquid chromatography. Alternative name: methyl cyanide; chemical formula: CH3CN acetonitrile in other Oxford dictionaries Definition of acetonitrile in the US English dictionary