Fusel alcohols, also sometimes called fusel oils, or potato oil in Europe, are higher-order alcohols (that is, alcohols with more than two carbon atoms) formed by fermentation and present in cider, mead, beer, wine, and spirits to varying degrees.^{[citation needed]}

The term fusel is German for "bad liquor".

The compounds involved are chiefly:^{[citation needed]}

Excessive concentrations of these fractions may cause off flavours, sometimes described as "spicy", "hot", or "solvent-like". Some beverages, such as whisky, Siwucha, and traditional ales and ciders, are expected to have relatively high concentrations of fusel alcohols as part of the flavour profile. In other beverages, such as Korn, vodka, and lagers, the presence of fusel alcohols is considered a fault.^{[citation needed]}

Fusel alcohols are formed when fermentation occurs:^{[citation needed]}

During distillation, fusel alcohols are concentrated in the "tails" at the end of the distillation run. They have an oily consistency, which is noticeable to the distiller, hence the other name "fusel oil". If desired, these heavier alcohols can be almost completely separated in a reflux still. Freeze distillation, on the other hand, does not remove fusel alcohols.^{[citation needed]}

An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils.

First attested in English 1176, the word oil comes from Old French "oile", from Latin "oleum", which in turn comes from the Greek "ἔλαιον" (elaion), "olive oil, oil" and that from "ἐλαία" (elaia), "olive tree". The earliest attested form of the word is the Mycenaean Greek e-ra-wo, written in Linear B syllabic script.

Organic oils are produced in remarkable diversity by plants, animals, and other organisms through natural metabolic processes. Lipid is the scientific term for the fatty acids, steroids and similar chemicals often found in the oils produced by living things, while oil refers to an overall mixture of chemicals. Organic oils may also contain chemicals other than lipids, including proteins, waxes and alkaloids.

Lipids can be classified by the way that they are made by an organism, their chemical structure and their limited solubility in water compared to oils. They have a high carbon and hydrogen content and are considerably lacking in oxygen compared to other organic compounds and minerals; they tend to be relatively nonpolar molecules, but may include both polar and nonpolar regions as in the case of phospholipids and steroids.