An existential risk (or x-risk) is a risk posing permanent large negative consequences to humanity which can never be undone such as the extinction of human civilization or event of similar severity, e.g. an eternal evil dictatorship.
In his seminal paper on the subject 1, Nick 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.