Glory Kill

A Glory Kill is a special action in Doom. These consists of the player performing a special melee action that instantly kills the enemy.

Description
Dealing enough damage to an enemy may cause them to 'stagger' which is indicated by a blue highlight. When the player comes into close range, indicated in an orange highlight, the player can perform a special melee action which kills the enemy instantly in a spectacularly brutal fashion. In addition, the player is invulnerable while performing the kill.

Boss battles (the Cyberdemon, the Hell Guards and the Spider Mastermind) can only be finished with glory kills; failure to do so will result in the boss recovering a bit of its health and continuing the fight.

Enemies killed through glory kills always drop health pickups.

In multiplayer, glory kill opportunities occur in random occurrences while dealing damage. Unlike glory kills in single player, the player is not granted brief invulnerability.

Trivia

 * Glory kills may have been inspired by a notably popular mod for the original Doom series called Brutal Doom, which included "Rip and Tear" mode, an alternate unarmed combat mode which could be accessed after grabbing a Berserk Pack, and which allowed you to perform Mortal Kombat-style fatalities on monsters.
 * At least two glory kills were cut from the official game release. One was a frontal glory kill on a Hell Knight, consisting of punching it in the head three times then snapping its neck. The other was a frontal glory kill on a Mancubus, consisting of having its core ripped out of its belly and having it rammed down its throat, whereas they briefly belch before exploding after a few seconds.