The Collaborative International Dictionary of English v.0.48:

Butyl \Bu"tyl\, n. [L. butyrum butter + -yl. See Butter.]
   (Chem.)
   A compound radical, regarded as butane, less one atom of
   hydrogen.
   [1913 Webster]

WordNet (r) 3.0 (2006):

butyl
    n 1: a hydrocarbon radical (C4H9)