Essential

Essential is an attribute which can be assigned to non-player characters in games which use the Gamebryo engine, including Fallout 3 and Fallout: New Vegas. The equivalent attribute for items is quest item, which should not be confused with the general usage of that term.

The main effect of this attribute is that the character cannot be killed, instead it will be temporarily rendered unconscious and wake up again later. In Fallout 3, all children in the game are essential characters, so even if you find a way to attack them, they still cannot be killed. This was done for legal reasons.

The purpose of this attribute is to prevent the player character from rendering quests uncompletable by killing characters which are necessary for that quest.

Fallout: New Vegas There are few essential NPCs aside from children. One of which is the robot Victor. Who is essential early on in the game. The message [Victor is unconscious] may appear during your early travels.

Exploits

 * If you have the requisite perk, essential NPCs generate a finger or ear each time they are knocked unconscious.
 * Essential NPCs can be targeted with the Mister Sandman perk an infinite number of times, generating unlimited XP. (Patched on all game versions)