Inuits may have the proverbial 50 words for snow, but futurists and forecasters are not far behind when it comes to labeling high-impact, low probability events. For a long time, it was acceptable to use phrases such as "wild card" to describe these factors which can unexpectedly influence or outright change the trajectory of events, such as an earthquake, radical technological breakthrough, pandemic, or sudden system failure.
The foresight community has worked with this idea long enough to have typologies and methods for sorting and comparing. Along came Nassim Nicolas Taleb to add another phrase to the pile with his book "The Black Swan: The Impact of the Highly Improbable," in 2007, which has dominated business and financial parlance throughout the Great Recession and recovery. The Game Changers project gave us X-Events (pdf) with their work in 2011. The National Intelligence Council's Global Trends 2030 report uses "game changers" (not to be confused with the project above) to refer to major uncertainties (slightly different, but same general function here). Now the World Economic Fund's 2013 Global Risk Report has thrown "X-factors" into the mix with the release of its annual study.
There have been a number of other terms used across professions, from foresight to risk management to defense. Though it was dismissed as a rhetorical flourish at the time, former US secretary of defense Donald Rumsfeld's "unknown unknowns" have experienced a resurgence in popularity recently in half-ironic nods to its efficacy as a descriptor for unanticipated events or factors. Responding to my comment about this issue, the helpful Twitterweb offered these additional terms for the pile, both serious and amusing.
Of course, these terms are not completely interchangeable, but some overlap heavily. An insurer's "Act of God" is a fund trader's "black swan" is a risk manager's "x-factor". It would be funny if it wasn't problematic (humorous submissions notwithstanding). With the "prospective community" so fragmented and not a little tribal, the proliferation of terms to describe such a critical component of strategic thinking is kind of scary—how do we value something we have a hard time agreeing the name of?
The pedant in me thinks there ought to be more taxonomic agreement so we are working with compatible terminology, even if we still have heterogeneous systems. Sadly, this is probably wishful thinking as the business consulting market rewards novelty over clarity (in the short term).
1/22/2013: A great piece was just posted by Helene Lavoix on Taleb and Black Swans. Recommended read.