1. define: capacity a) "heaviness". the downward force caused by gravity on an object. a measure of mass in the us and imperial measurement systems. approximate weight as a slice of bread. it is exactly 28.349523125 grams b) the amount that something can hold. usually means volume, such as milliliters (ml) or liters (l) in metric, or pints or gallons in imperial. c) a measurement of volume in the us and imperial measurement systems. about how much fits into a small medicine cup. 8 fluid ounces = 1 cup.