We shall use the following four categories to classify existential risks:
Bangs – Earth-originating intelligent life goes extinct in relatively sudden disaster resulting from either an accident or a deliberate act of destruction.
Crunches – The potential of humankind to develop into posthumanity is permanently thwarted although human life continues in some form.
Shrieks – Some form of posthumanity is attained but it is an extremely narrow band of what is possible and desirable.
Whimpers – A posthuman civilization arises but evolves in a direction that leads gradually but irrevocably to either the complete disappearance of the things we value or to a state where those things are realized to only a minuscule degree of what could have been achieved.
Nick Bostrom. March 02002.