Newbie question - What is balance?

Hi,

I have recently started using Metis to partition some sparse graphs with weighted edges. When I run the software it gives me two values: edge-cut and balance. I understand what edge-cut is, but I could not find a precise definition of "balance" in the manual. I assume it refers to the sizes of the partitions being similar, however I would like an exact definition and want to know how it is computed. Thank you for your help.