Diacetyl (C4H6O2): a natural byproduct of fermentation, often present in alcoholic beverages like beer and wine that can also be synthesized artificially. Diacetyl is used as a food flavoring to produce a buttery flavor and aroma. Food flavorings containing diacetyl are used in microwave popcorn and other snack foods, pet foods, candies, baked goods, and other food products. Diacetyl is also called butanedione or 2,3-butanedione. High concentrations of diacetyl (diacetyl vapor) has been recently implicated in a series of severe respiratory illnesses (bronchiolitis obliterans or "popcorn lung") affecting microwave popcorn plant workers and a small number of consumers.