The amount of gas, liquid, or material that a single balloon can hold or contain.
From 'balloon' plus the suffix '-ful' (from Old English 'full'), denoting a quantity that fills or satisfies one unit of the container.
A balloonful of helium at sea level weighs almost nothing and lifts about 1 gram, but at high altitudes where the air is thinner, that same balloonful creates powerful lift for weather balloons.
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