In reply to Rog Wilko:
> I keep hearing this phrase - lots of threats about at the moment, of course. It seems to have first appeared on the media about a year ago. Just what does existential mean? I know it is (or was) an -ism, associated with Sartre & de Beauvoir, but this is clearly a totally different usage. Does it just mean existing? If so, totally redundant.
Not sure that it is redundant. It is usually used in relation to a particular entity; often a state. An existential threat might be a foreign enemy, weapon, or internal movement that could cause the state to cease to exist in any recognisable form. In contrast, a non-existential threat might have the capability to kill many citizens, harm the economy etc, but would not threaten the existence of the state as a functioning entity.