|1.||pertaining to existence.|
|2.||of, pertaining to, or characteristic of existentialism: an existential hero.|
So when you hear some Neocon bobblehead yawping about how so-and-so is "an existential threat," keep this in mind. He's saying one of two things:
1) a group of criminals who like to blow up things threatens the very existence of the United States on this planet; or
2) a group of criminals who like to blow up things are stressing the individual's unique position as a self-determining agent responsible for the authenticity of his or her choices.
It is left as an exercise for the reader to determine whether either proposition makes any kind of rational sense whatsoever.