Essential

"Essential" is an attribute which can be assigned to non-player characters in games which use the Gamebryo engine or one of its derivatives, such as the in-house Creation Engine. Fallout 3 and Fallout: New Vegas use the original Gamebryo engine, with Fallout 4 utilizing the Creation Engine. 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 later Fallout games, all children are essential, but they are different from other NPC's marked with the attribute in the fact that they are also coded to not take damage from any source, thus they cannot be knocked unconscious or killed. Children can be still attacked, but they will always cower and flee instead of instead retaliating against the player character, and cannot be targeted in VATS. Children were made essential for the sake of "social responsibility," according to Emil Pagliarulo.

The main purpose of this attribute is to protect the integrity of certain quests which do not account for the death of an NPC important to the plot of the quest. This prevents the player character from breaking the quest and making it impossible to complete by killing an important NPC. However, there are still characters involved in quests that can be killed, but most of the time their deaths will result in the quest being failed or proceeding differently. The completion of certain quests or tasks will make the NPCs associated with them able to be killed, though most characters integral to the main plot of their respective game are essential permanently throughout gameplay. Sometimes the death of an NPC associated with a quest will result in a pop-up message informing the NPC in question has died, with the quest subsequently failing.

In Fallout: New Vegas, the only NPCs aside from children that are essential during gameplay are Companions in the player character's party (unless playing Hardcore mode).

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.