A defense. Can be provided by various game abilities or plot created scenarios. From the Accelerant core rules book:

"You are affected by the attack in question, but the effect has been mitigated, lessened, or changed entirely. The actual reduction is determined by the skill or ability that allows you to use this defense. Reduce is often used to indicate the effect has been lessened. The verbal for this defense might be followed by "to" and the modified effect such as "Reduce to Maim". You must role play the new effect appropriately."

This effect is only slightly different than an Absorb. It is generally used to take a penalty and make it less severe. A good example is reducing a Death Effect to Vitality loss and becoming Unstable.