Invulnerability

The invulnerability is a powerup that makes the player generally impervious to damage for 30 seconds, found in all classic Doom games. The player is not protected from telefrags, and will still recoil from a successful attack. The powerup looks like a green sphere with a grinning red-eyed head inside. While it is active, the colors in the player's display change to inverse monochrome, and the eyes of the status bar face glow yellow (STFGOD0), making them look similar to the Demon's; in the last few seconds the display will begin blinking back to its original colors. While the inverted monochrome effect is activated the game is effectively fully bright, and the player will be able to see every detail even in the darkest areas. Spectres in particular become much easier to see under its effects.

Invulnerability can also be activated with the cheat code idbeholdv.

In deathmatch mode of vanilla Doom, invulnerabilities and blur artifacts never respawn. Some source ports allow flags to be set to cause them to respawn.

Invulnerabilities are included in the items percentage displayed at the end of each level.

Technical
According to the function P_DamageMobj in p_inter.c, invulnerability only protects the player from attacks doing less than 1000 points of damage; while no monster in the game can do nearly that much damage in one attack (a direct hit from the rocket of a Cyberdemon will inflict at most 288 damage), a telefrag inflicts 10000 points (and indeed will kill even an otherwise invulnerable player). This also means that DeHackEd can be used to circumvent the invulnerability powerup by creating incredibly powerful weapons.