Magic Potency

From FFXI Wiki

Magic Potency is a direct increase to the amount of damage a spell or magical weapon skill/ability of the corresponding type will do. For example, a 10% increase in fire potency will cause the spell Fire IV to do 10% extra damage, but will also cause the weapon skill Wildfire and the job ability Fire Shot to do 10% extra damage.

Magic potency for specific elements is directly meritable in BLM group 1 merits, with a 2% damage increase with each upgrade.