The average voltage of the battery pack throughout the discharge. It varies between the different brands and models, but within the same chemistry group, the differences are insignificant. Still, higher average voltage for the same capacity means more energy (in Watt-hours). This can be calculated as Uavg×Capacity and is displayed in the detailed page for each battery.
Stricly speaking, given the nature of the test, a more accurate computation of the energy would be to integrate V2/R over time; however, I did some test calculations in that direction, and didn't notice a significant difference.