Nassim Nicholas Taleb uses the term Black Swan to identify significant unexpected events. Holling made some similar points from a different perspective in his 1973 paper on resilience and his 1986 paper the resilience of terrestrial ecosystems; local surprise and global change. In on the interdisciplinary Edge Taleb writes on Learning to expect the unexpected and defines what he means by Black Swans:
A black swan is an outlier, an event that lies beyond the realm of normal expectations. Most people expect all swans to be white because that’s what their experience tells them; a black swan is by definition a surprise. Nevertheless, people tend to concoct explanations for them after the fact, which makes them appear more predictable, and less random, than they are. Our minds are designed to retain, for efficient storage, past information that fits into a compressed narrative. This distortion, called the hindsight bias, prevents us from adequately learning from the past.
From my perspective, Black swans occur when there are significant mismatches between the models people use to understand the world and the subsquent expectations that those models produce and observations. In other words, black swans are model errors – something that I’ve written (Peterson, Carpetner & Brock et al 2003) in the context of ecological management.