Tail risk
\ teɪl \ rɪsk \
In statistical theory, tail risk is the risk of an extreme or unlikely event.
A tail event is the occurrence of a such and extreme event. Technically, the event could have a positive or negative effect, but risk management practices tend only to focus on negative tail events, known as downside tail events.
A tail event is often defined as one with less than a 1 in 200 chance of happening. However, it is extremely difficult to calculate precise probabilities of such extreme events in practice.
In longevity terms, examples of potential tail events are a cure for aging or a severe climate event. Although, which one you view to be a downside tail event depends on your perspective.