Weapons can be outfitted with nano-effects. These powerful effects can trigger any time a target is struck by the weapon. Common nano-effects include bio, incendiary, electric, radioactive, and syphon.
Nano Effects Symbols
(Credit for image below goes to Jaxxas from Defiance Official Forums)
EFFECT: Increases damage done to the affected target by 20%
EFFECT: Stuns target and enemies near target, also shorts out HUD.
EFFECT: Burns the enemy, dealing damage over time. Also, causes NPC's to panic.
EFFECT: Slows enemies movement and decreases their damage dealt.
EFFECT: Makes target vulnerable and increases the damage they take.
EFFECT: When the effect is triggered, it will heal the players HP, or shield should the player have max HP. When the effect occurs you will hear a sound and a few seconds later around ~450 damage will be applied to the target and you will be healed.