Diacetyl (IUPAC systematic name: butanedione or 2,3-butanedione) is a natural byproduct of fermentation. It is a vicinal diketone (two C=O groups, side-by-side) with the molecular formula C4H6O2. Diacetyl occurs naturally in alcoholic beverages and is added to some foods to impart a buttery flavor.
Diacetyl and acetoin are two compounds that give butter its characteristic taste. Because of this, manufacturers of margarines or similar oil-based products typically add diacetyl and acetoin (along with beta carotene for the yellow color) to make the final product butter-flavored, because it would otherwise be relatively tasteless.