Source : Webster's Revised Unabridged Dictionary (1913)

Fusel \Fu"sel\, n., Fusel oil \Fu"sel oil\ [G. fusel bad
   liquor.] (Chem.)
   A hot, acrid, oily liquid, accompanying many alcoholic
   liquors (as potato whisky, corn whisky, etc.), as an
   undesirable ingredient, and consisting of several of the
   higher alcohols and compound ethers, but particularly of amyl
   alcohol; hence, specifically applied to amyl alcohol.

Source : WordNet®

fusel oil
     n : a mixture of amyl alcohols and propanol and butanol formed
         from distillation of fermented liquors