A view that the probabilities of very large market movements - positive or negative - are greater than predicted by a simple random walk model of market prices.
In other words, the view that market shows trending behaviour in relation to large market movements: 'Both panic and over-exuberance are contagious'.
This view is reflected in deeply out-of-the money options having greater implied volatilities (calculated from their greater traded values) than at-the-money options.
This view is also a potential explanation for leptokurtic frequency distributions. (Which have longer 'tails' of extreme values than the simpler Normal frequency distributions often used for modelling and calculation purposes.)