Where people expect financial investments to rapidly increase, like in the economic boom of the eighties. It is so called, because masses of first-time investors rush head-first into the markets with thier money, expecting to amass riches beyond belief.
Japan's 1980's bull market was so good that, in 1989, Japan was expected to overtake the United states as the world's richest economy. The reccession of the 90's prevented this.
Today, it is just recovering from a bear market