Noun
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: Webster's dictionaryIt can be separated from fusel oil by either of two methods: shaking with strong brine solution and separating the oily layer from the brine layer; distilling it and collecting the fraction that boils between 125 and 140 °C. Source: Internet
The high molecular weight of alcohol compounds (fusel oil) is what contributes to hangovers. Source: Internet