A quart is a unit of measurement for liquids, which refers to one-fourth or 25% of a larger container. In other words, it's four-fifths (4/5) of a gallon, eight-ninths (8/9) of a barrel, sixteen-sixteenths (16/16) of a keg, and so on. The word "quart" comes from the Old French word "cort," meaning "short."