Essential

Essentiality is an attribute which can be assigned to non-player characters in games which use the Gamebryo engine, including Fallout 3, Fallout: New Vegas, and Fallout 4. 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 both games, all children in the game are essential characters, so even if a way to attack them is found, they still cannot die.

The purpose of this attribute is to prevent the player character from rendering quests incompletable by killing characters who are necessary for that quest. However, there are still characters involved in quests that can be killed. Upon their death, a notification will pop up saying that the quest they were involved in has been failed. However, the failed quest will not be crossed out in the Quests screen.

In 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 later in the game. All Companions are essential (unless playing Hardcore mode). Yes Man is also one of the very few essential characters, except for the fact that he could be viewed as just "respawning."

During the quest G.I. Blues, Orris will shoot some Freeside thugs. The shots are scripted to inflict immense fatigue damage on the thugs, which will knock them unconscious as if they were essential.

Children were made essential for the sake of "social responsibility", according to Emil Pagliarulo.

Exploits
Having the Lawbringer or Contract Killer perk, essential NPCs generate a finger/ear each time they are "killed".

Bugs
Fallout: New Vegas: random creatures may be tagged as essential, particularly golden geckos and creatures in their vicinity, effectively making them unstoppable.