Defence prevents attacks from hitting you, or makes a hit less severe.
All defence dice are rolled simultaneously at the beginning of an encounter. Certain memories, especially ones that grant move, can let you reroll some or all of your defence dice during combat.
When you roll a number on a defence die, you normally do not receive damage on that Aim. For example, if you have two dodge defence dice from your Vitality, and you roll a 10 and a 6, then you can’t be hit on a 10 or a 6.
Defence primarily comes in three flavours; Cover, Dodge, and Block.
Cover
When a unit is between two other units, it grants Cover to both those units, relative to each other.
Cover Defence is rolled by the DM when a unit is between you and a foe, and an attack is made. If an attack is on an aim which was rolled be the cover defence, it hits the cover unit instead of the target.
If multiple attacks are made in the same round and the same unit is granting cover on more than one of them, cover defence isn’t normally rerolled. However, it may be if the unit granting cover is moved or damaged.
Hiding
When you are behind cover and adjacent to cover of a sufficient size, you are hidden. This gives you an extra attack die in combat against foes on the other side of that cover. Moreover, it gives -1 to Insight and Resolve to both units on information checks regarding the opposing unit.
A cover unit hides you if it is equal or greater to your size.
Dodge
Dodge is an instinct which most creatures and characters have.
Dodge grants you Defence dice equal to your Vitality.
You roll Dodge at the beginning of a combat encounter, and may usually reroll it when you move.
Block
Many memories can be used as a block. If this is the case, it will read “Block(1)” in the description.
If your memory is a block, it is immune to damage. That is, it can’t be replaced by fight or flight when you are dealt normal damage. Instead, the memory blocks that attack.
A blocked attack still hits. In other words, a blocked memory might be replaced by fight or flight if it is the target of a control attack, or if the attacking foe has concussive damage.
Some memories might read “Block(2)” or some higher integer. If this is the case, this memory can block for itself *and* another memory, including one which isn’t a block.
Armour
Armour is a special type of block. Armour usually grants block on specified aimlines. For example, you may have a breastplate that blocks all Aim (8 or 9) attacks. This is noted in the memory description as “Armour(8-9)”.
If you have a piece of Armour that blocks a specific aimline, you can’t normally have another piece of armour that blocks that same aimline.