The Collaborative International Dictionary of English v.0.48:

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 mostly of amyl
   alcohol; hence, amyl alcohol or a mixture of amyl alcohols.
   [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Alcohol \Al"co*hol\ ([a^]l"k[-o]*h[o^]l), n. [Cf. F. alcool,
   formerly written alcohol, Sp. alcohol alcohol, antimony,
   galena, OSp. alcofol; all fr. Ar. al-kohl a powder of
   antimony or galena, to paint the eyebrows with. The name was
   afterwards applied, on account of the fineness of this
   powder, to highly rectified spirits, a signification unknown
   in Arabia. The Sp. word has both meanings. Cf. Alquifou.]
   1. An impalpable powder. [Obs.]
      [1913 Webster]

   2. The fluid essence or pure spirit obtained by distillation.
      [Obs.] --Boyle.
      [1913 Webster]

   3. Pure spirit of wine; pure or highly rectified spirit
      (called also ethyl alcohol or ethanol, CH3.CH2.OH);
      the spirituous or intoxicating element of fermented or
      distilled liquors, or more loosely a liquid containing it
      in considerable quantity. It is extracted by simple
      distillation from various vegetable juices and infusions
      of a saccharine nature, which have undergone vinous
      fermentation.

   Note: [The ferementation is usually carried out by addition
         of brewer's yeast, Saccharomyces cerevisiae to an
         aqueous solution containing carbohydrates.]
         [1913 Webster +PJC]

   Note: As used in the U. S. "Pharmacop[oe]ia," alcohol
         contains 91 per cent by weight of ethyl alcohol and 9
         per cent of water; and diluted alcohol (proof spirit)
         contains 45.5 per cent by weight of ethyl alcohol and
         54.5 per cent of water.
         [1913 Webster]

   4. (Organic Chem.) A class of compounds analogous to vinic
      alcohol in constitution. Chemically speaking, they are
      hydroxides of certain organic radicals; as, the radical
      ethyl forms common or ethyl alcohol (C2H5.OH); methyl
      forms methyl alcohol (CH3.OH) or wood alcohol; amyl
      forms amyl alcohol (C5H11.OH) or fusel oil, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fusel oil
    n 1: a mixture of amyl alcohols and propanol and butanol formed
         from distillation of fermented liquors