An existential risk (or x-risk) is a risk that poses astronomically large negative consequences for humanity, such as human extinction or permanent global totalitarianism.

Nick Bostrom introduced the term "existential risk" in his 2002 paper "Existential Risks: Analyzing Human Extinction Scenarios and Related Hazards."¹ In the paper, Bostrom defined an existential risk as:

One where an adverse outcome would either annihilate Earth-originating intelligent life or permanently and drastically curtail its potential.

The Oxford Future of Humanity Institute (FHI) was founded by Bostrom in 2005 in part to study existential risks. Other institutions with a generalist focus on existential risk include the Centre for the Study of Existential Risk.

FHI's existential-risk.org FAQ notes regarding the definition of "existential risk":

An existential risk is one that threatens the entire future of humanity. [...]
“Humanity”, in this context, does not mean “the biological species Homo sapiens”. If we humans were to evolve into another species, or merge or replace ourselves with intelligent machines, this would not necessarily mean that an existential catastrophe had occurred — although it might if the quality of life enjoyed by those new life forms turns out to be far inferior to that enjoyed by humans.

Classification of Existential Risks

Bostrom² proposes a series of classifications for existential risks:

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