Overrun is the term for the amount of air incorporated into ice cream during churning and freezing.
As ice-cream mix freezes, a dasher or other agitator introduces air while also controlling the growth of ice crystals. The increase in volume caused by that air is called overrun. It is commonly expressed as a percentage comparing the volume of the finished ice cream with the volume of the original mix.
For example, 100 percent overrun means the finished product has twice the volume of the liquid mix because an amount of air equal to the mix’s original volume has been incorporated. Products with high overrun feel lighter, while products with low overrun tend to be denser and more concentrated.
Overrun is not simply a measure of quality. Different styles are designed with different amounts of air, and legal or industry definitions vary by country. Gelato generally has less overrun than many American-style ice creams, but the exact figure depends on the recipe and equipment.