A Smart defense to negate an attack and throw it back at the attacker. Can be provided by various game abilities or plot created scenarios. From the Accelerant core rules book:
"You choose one melee, missile, or packet attack of the appropriate type to rebound back at the attacker when it strikes you. Call this defense to negate the effects of the attack. If your attack is reflected then you must take the attack as if your melee, missile or packet had struck you. The attack retains all of the original traits so it might not actually affect you. You may use defenses to negate the attack, including using Reflect to bounce it back on the original target."
So if you attacked a creature with something you were personally unaffected by, and it reflected at you, nothing would happen. For example a reflected "Death to Undead" would not be a problem for a character with the Living trait.
It is in theory also possible for one effect to "volley ball" between characters if they each had enough reflects to do so. Eventually someone will run out of reflects however, and either take the attack or use some other defense to mitigate it.