SRD:Light Weapon

Light Weapons
A light weapon is easier to use in one’s off hand than a one-handed weapon is, and it can be used while grappling. A light weapon is used in one hand. Add the wielder’s Strength bonus (if any) to damage rolls for melee attacks with a light weapon if it’s used in the primary hand, or one-half the wielder’s Strength bonus if it’s used in the off hand. Using two hands to wield a light weapon gives no advantage on damage; the Strength bonus applies as though the weapon were held in the wielder’s primary hand only.

An unarmed strike is always considered a light weapon.