A compound existential risk (also known as a combination existential risk (Brennan 2019) and as an intermediate existential risk (Ćirković, Sandberg & Bostrom 2010)) is an existential risk arising from two or more distinct events.

Owen Cotton-Barratt, Max Daniel and Anders Sandberg illustrate the concept of a (non-existential) compound risk with the following historical incident (Cotton-Barratt, Daniel & Sandberg 2020: 279):

the deadliest accident in aviation history occurred when two planes collided on an airport runway; this was only possible because a previous terrorist attack on another airport had caused congestion due to rerouted planes, which disabled the prevention measure of using separate routes for taxiing and takeoff.

The events whose conjunction constitute the compound existential risk may, but need not, themselves be existential risks. For example, climate change may pose a small existential risk in and of itself but, in addition, pose a compound risk by triggering an AI race, which would arguably have been less likely to occur in the absence of disruptions to the climate system (Brennan 2019).