A cup is a unit of volume used in recipes. Its size depends on the measurement system: a US customary cup is about 236.6 milliliters, often rounded to 240 milliliters in recipe conversions, while a metric cup is 250 milliliters. Check the recipe's convention rather than assuming all cups are interchangeable.
A measuring cup is different from an ordinary drinking cup. For dry ingredients, use the filling method specified by the recipe; flour is commonly spooned into the cup and leveled without packing. A cup measures volume, not weight, so a cup of flour and a cup of sugar do not weigh the same amount.
Rice-cooker cups can also differ from recipe cups. The cup supplied with many rice cookers, including Zojirushi models, holds about 180 milliliters. Use the supplied cup with the matching markings in the cooking pot and follow the appliance manual.