Area damage, or splash damage, is an attribute that specifies how far the unit's attack is applied. Area damage is usually on siege and bombers. The more area damage the unit has, the blast will have a bigger radius, damaging multiple units. As with most attributes, this one may be upgraded, however it's more expensive than the others.
The values for area damage do not have the same scale as range. 1 range is equivalent to one "block" on the map's terrain, whereas 1 area damage is equivalent to only one-tenth (1/10) of a block. These are the radial values, so for example, a Bronze Cannon has an area damage of 15, which covers a distance of 1.5 blocks in all directions from the point of impact, for a total damage width of 3 blocks. However, units that are farther from the point of impact receive less damage.