Syntax: defense [random/roll/dodge/block]
You can set your defense to random (default), roll, dodge or block. Block, dodge and roll have their own strengths and weaknesses (detailed below), and will be more or less effective depending on the enemy you are fighting.
Block is based primarily on strength, though wisdom and constitution are also considered. Block is more effective against piercing attacks, though it is less effective against blunt weapons. Block will also reduce damage at a much greater rate than the other two forms.
Dodge is based primarily on dexterity, though wisdom and intelligence both factor prominently. Dodge is more effective against blunt attacks, and less effective against slashing attacks. Dodge will negate damage if it succeeds, though if you fail to dodge, you will take the full amount.
Roll is based primarily on quickness, though wisdom and dexterity are also taken into consideration. Roll is more effective against slashing attacks, and less effective against piercing attacks. Roll will negate damage if successful, but will reduce damage if it fails. The damage Roll prevents on a failure is much less than that of a success, but Roll also has a lower chance of avoiding the blow than dodge.
Random isn't an actual defense, so much as it randomly chooses one of the three forms of defense for you every attack. Furthermore, you cannot improve your defenses when using random, because you're not focussed enough to do new things. You can still learn from watching your opponent, though.