The Shield is a power-up item that made its debut in Vigilante 8. The Sheild power-up will make you invincible to all enemy attacks for a short amount of time, hence the name. They are a bright green diamond shaped object found randomly on any stage in Vigilante 8, Vigilante 8: 2nd Offenseand Vigilante 8: Arcade.
In Vigilante 8: 2nd Offense, if your vehicle catches fire before picking up a Shield, you still take damage from the fire. Besides, some Specials like Houston's Tow Twister and Garbage Man's Compactor (if the vehicle is compacted inside) can deal damage to vehicles that have this power-up active.