A Smart defense to end an attack or effect. Can be provided by various game abilities or plot created scenarios. From the Accelerant core rules book:

"You may negate an attack after it has affected you. You must spend three seconds to role play this defense as you shake off the effect, during which time you must remain relatively still. You cannot use game skills while role playing the purge. You cannot use Purge while unconscious unless doing so would wake you. You cannot use Purge when you are dead unless the skill specifically allows you to use “Purge Death” in the verbal."

The primary difference between this defense and a defense such as Shield or Avoid is that the effect actually hits you prior to you ridding yourself of it. Other defense types allow you to act as though the attack never even happened - this is not one of those defenses.