Parry Ability skill
The Parry Ability skill is what dictates your character's ability to fend off an incoming melee or pole-ranged attack with the weapon in your hand.
Parry is affected by Reflexes, Agility, and the current weapon you have in hand. Attempting to parry with a poorly balanced weapon or a weapon in which you have few ranks in will be less successful than parrying with a well balanced weapon or one you have many ranks in.
Parrying also teaches some of the skill of the weapon you are parrying with. Example: If one were to parry with a longsword one would gain experience in the Heavy Edged skill in addition to Parry.
One other interesting note is that both Paladins and Barbarians can knock aside bows if the shooter fires within the range of the parrying weapon. To be successful they must have some defensive stance points allocated to parry.