Usually refers to cycles of buying and selling en masse particularly in the stock market. Some of these cycles seem to be predictable and based on factors such as the time of year, while others, such as ‘bubbles,’ are harder to detect or predict.
Some people believe that it is the nature of modern capitalism to have boom-bust cycles. A cycles starts with a ‘bubble,’ where irrational exuberance leads to prices that are far beyond the real value of the thing traded. Bubbles are followed by busts where exuberance is replaced by fear.