Subtract your character’s Defence from any unarmed, melee, or thrown attacks of which the character is aware.
Every time your character applies his Defence against an attack, reduce his Defence by –1 until the start of the next turn.
Spending a point of Willpower increases her Defence by +2, but only against one attacker.
You can choose not to apply your character’s Defence against some attacks. If two unarmed gangbangers attack before a chainsaw-wielding lunatic, you might want to let the gangbangers get their blows in, and apply your full Defence against the maniac swinging a chainsaw at your head.
You cannot apply your character’s Defence against firearms attacks without supernatural assistance, whether it’s from a spell, an Attainment, or some other power that grants Defence against gunfire.
(I left the word “gangbangers” in because I find it hilariously dated, somehow this game was released in 2016)