Anyone you see in a game, on TV, or in real life that you don't know enough about to care about. A character that's there just to fill up space; there's no chance that the DM has actually created a backstory for them. For example, people you see on the subway, or the new member of the away team from Star Trek. Of course, in real life, Everyone is a PC; but the brain gets lazy and stereotypes strangers, so you don't have to worry about them.