Fusel alcohol
Fusel alcohols or fuselol, also sometimes called fusel oils in Europe, are a mixture of several alcohols (chiefly amyl alcohol) produced as a by-product of alcoholic fermentation. The word fusel is German for "bad liquor".
Fusel alcohols refer to alcohols that contain more than two carbons. Ethanol (the good stuff) is a two carbon atom molecule. They happen naturally during all fermentations, with higher concentrations occurring when the yeast are under stress (including high temps).
In distilling Fusel oils express themselves in the tails. Fusel alcohols are also a component to Ester production.