An enemy status effect, in which the spawn will reduce damage taken for a number of turns by a specific percentage, usually ranging between 50% and 99%. Unlike enemy defense, using a defense void active does not allow you to bypass an enemy shield. The only solutions for getting around this mechanic are to either stall until the shield goes away, or to simply deal more damage to the spawn.
Shields allow for spawns to exceed the normal enemy hp cap of 2.1 billion (the same as PAD’s damage cap), and spawns that do this can be extremely annoying, such as Amon
from an old monthly Challenge 9, who had 8 billion effective HP, due to a 75% shield on top of 2 billion normal hp, or the most extreme example of them all, Mega Awoken Dark Kali
from Annihilation Endless Corridors who has a 999 turn 99% shield, on top of her 250 million normal hp, meaning she has a whopping 25 billion effective hp, or around 12 damage capped attacks.