serial killer

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A person who commits multiple (more than two) murders, especially similar ones with no obvious motive over a period of time with a "cooling-off" period between each murder.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun someone who murders more than three victims one at a time in a relatively short interval