Essential NPCs: Non player characters that are incapable of dying, but instead when health gets to low usually faint for a few seconds and then return to consciousness with slightly recovered health.
There were some NPCs in FO3 that really irrated me when they died, like the caravan traders that appeared infront of settlements. They actually would spawn in active world cells near you with out you noticing them and get killed by raiders. which was very annoying as i wanted to dump my crap on them for caps. but other than that there are few NPCs that I want to be immortal.