Many criminologists consider the term to mean someone, who kills repeatedly, usually for sex.
But serial killers are not just limited to sex crimes. They can include maniacal killers and those who kill for money or valuables. The reason why criminologists almost always associate serial killers with sex crimes is because most experts believe that sexual urges direct the serial killer's crimes. David Lester, Ph.D. devotes a section of his book to defining serial murder as well. There is more about Lester and his theories below.