Strength

From Fire Emblem Wiki

Strength is a stat that affects the amount of damage you do when using a non-magic attack. In Fire Emblem: Path of Radiance and Fire Emblem: Radiant Dawn it also affects a unit's attack speed.

Strength is one of the two contributing factors to Attack Power, the other being the Might of the weapon being used. The higher the strength a unit possesses, the more damage that unit will deal.

In Path of Radiance and Radiant Dawn, Strength is also used to calculate Attack Speed. The higher a unit's strength, the heavier the weapon can be before the unit suffers an attack speed penalty.