Existential Risk

Also known as an x-risk, an event capable of destroying a civilization either by killing its members or disrupting it so much it cannot continue in its present form. The omega symbol (above) is often used by humans and related species to represent the concept since it is the final letter of the Ancient Greek alphabet.

Taxonomy of Types

While low-probability existential risk threatens every planetary civilization in the form of asteroid impacts, gamma ray bursts, nearby supernovae, etc., researchers consider self-caused or endogenous risks created by civilizations themselves as the greater threat.
A very common type of x-risk is one which emerges as a consequence of a new technological innovation, such as the invention of nuclear weapons by humans leading to a military standoff in the Nuclear Dark Days which threatened to obliterate their civilization. Others are a by-product of an otherwise-desirable activity. Climate change on Terra was a direct result of consumer capitalism, which also lifted many people out of poverty.
Existential risks can even be deliberately created, as in the case of a bioterrorist engineering a hyper-infective disease.
A first contact scenario can also qualify as an existential risk in the case of a "Christopher Columbus Scenario" where technologically advanced visitors introduce ideas or technologies which disrupt the fabric of a less-advanced civilization or even outright attack it.
In certain cases an existential threat can actually be beneficial to some groups, for example in the case of a popular revolt which deposes a tyrannical regime and replaces it with a democratic government.

Management and Reduction

Decreasing the threat posed by an existential risk is hard, preventing a new one from emerging is even harder since even totalitarian governments have trouble restricting innovation and access to techology.
The generally accepted way to reduce existential risk is careful regulation. The precautionary principle is paramount here, governments should ensure new technologies are safe before allowing their deployment and give agencies the proper powers to rein in abuses, practicing proactive rather than reactive regulation. Systemic factors can work against x-risk reduction, for instance the Nuclear Dark Days would have never happened if the nations of Earth agreed to disarm themselves of nuclear weapons, but if they did so there would be an immediate incentive to reacquire them for security and power projection. If one nation did so, the other nations would have to follow suit or risk being blackmailed into submission. Some political ideologies (e.g. American-style libertarianism) also consider regulation anathema, regardless of consequences. For this reason the concepts of existential risk and multipolar traps are fundamentally linked.

Instances of Existential Catastrophe

• European contact of Native Americans (marginalization and extermination by a technologically superior culture)
  
• Terran Collapse (ecological overshoot followed by fall of civilization)
• Polganite Terminus Wars (peak oil and nuclear apocalypse)
• Age of Strife (relativistic interstellar conflict)