Mass hysteria is when irrational or wild and uncontrollable behavior spontaneously appears and spreads rapidly through a crowd or through a section of society.
Two of the most famous cases of mass hysteria are the Salem witch trials in seventeenth century Massachusetts, and the mass panic that spread in a number of American cities in 1938 during a radio broadcast of Orson Welles' adaptation of H. G. Wells' novel The War of the Worlds (it was about the invasion of earth by Martians--many people who tuned in the middle of the broadcast thought it was a real news report).
On the other hand, movements such as the Nazi movement in Germany and Heaven's Gate wouldn't count as mass hysteria--they were well thought out and intentional chronic pathogenic memes.