Sometimes called the flange diameter and sometimes called the PCD or pitch circle diameter. It's the distance from the centre of the hub spoke hole to one that is diametrically opposite. It's usually the same for both flanges but sometimes it is worth checking just to make sure. We tend to assume that the flange is perfectly round and has been drilled perfectly too but just to get a more accurate measurement do it a few times just to be consistent.
Part of the process of calculating the spoke length required for a wheel build is the measurement of the hub hole diameter. This is usually between 2.3 and 2.9 mm. An error in your measurement by 0.1 mm will not effect your calculation noticeably.