What is a Serial Killer?

Where did the term serial killer originate? I went to the FBI webpage to look up some facts regarding what constitutes a serial killer.

Here's what I found out:

• Term refers to three or more killings over a period of time
• Differs from mass murder which is killing four or more during one incident
• Differs from spree murder because between serial murders there is a 'cooling off' period
• They exhibit one of more of the MacDonald triad
• Fire setting
• Bed wedding beyond age of 12
• Cruelty toward animals
• They may be organized or disorganized
• There is some discrepancy on who actually coined the phrase 'Serial killer'. Some state it was FBI investigator, Robert Ressler. Some state it was  LAPD detective Pierce Brooks, creator of the ViCAP system
• Holmes & DeBurger categorize serial killers into 4 types:
• Power/Control-Want to exercise control over their victim. The power is the high they're seeking.
• Visionary-may have voices in their heads willing them to kill. They usually suffer from mental illness
• Hedonist-Kill because they like it, and it serves a purpose in their life
• Missionary-Want to rid the world of a particular group of people they consider undesirable

For more information, check out the FBI website: http://www.fbi.gov/stats-services/publications/serial-murder