The term mass murder refers to when a murder of four people or more happens. Usually the murders happen at the same time or over a relatively short period of time. The FBI defines mass murder as the killing of many people without any cooling off period. By comparison a serial killer has a cooling off period between murders. Murder can be done by a person, a terrorist group, a government or police. In many cases, the killer has a mental disorder. In many cases, the killer commits suicide or is killed by police.
The University of Texas massacre in August 1966, the 1999 Columbine High School massacre, the Virginia Tech massacre, the 2012 Aurora shooting and the 2012 Sandy Hook Elementary School shooting are some examples of mass murders that have happened in the United States.