butylPronunciation: /ˈbjuːtʌɪl, -tɪl/Definition of butyl noun [as modifier] Chemistry of or denoting an alkyl radical C4H9, derived from butane:butyl acetateSee also isobutyl. short for butyl rubber.Origin: mid 19th century: from butyric acid + -ylbutyl in other Oxford dictionaries Definition of butyl in the US English dictionary