A customary unit used to measure capacity.
Informally, a quantity that roughly fills a standard-sized coffee cup. Formally, a unit of capacity (volume) equal to exactly 8 fluid ounces. It can be used as a measure of a volume of liquid or as a dry measure (e.g., a cup of flour). In either case, it refers to the same volume.
|1 cup||8 oz (8 ounces)||24||20|
|1 pint||2c (2 cups)||25||21|
|1 quart||4c (2 pints)||26||22|
|1 half gallon||8c (2 quarts)||27||23|
|1 gallon||16c (2 half gallons)||28||24|
For more detail, see "Cup" on Rowland's Dictionary of Units of Measurement.
Notice that this system of measurements is based entirely on doubling: A tablespoon doubled, doubled again, and doubled a third time is a half-cup. Double again to get a cup; again to get a pint; again to get a quart; again to get a half-gallon; and again to get a gallon.