Serial Killer: A Construction

Why are serial killers called serial killers? This is a question a i had in my head when i was in a bus about to go home. Is there a specific reason in why they would name a killer or murderer a "serial killer", I mean why not just call it killer or murderer or crazy knife stabbing guy, So i looked it up and found an article entitled
Serial Killers Introduction by Maria on twistedminds.creativescapism.com. She said that the original term "Serial Killer" came from a former FBI special agent and one of the founding member's of Bureau's elite science unit aka "The Mind Hunters" or "The Psyche Squad", Robert Ressler:

In his 1992 memoir,Whoever Fights Monsters, Ressler writes that, in the early 1970s, while attending a weeklong conference at the British police academy, he heard a fellow participant refer to “crimes in series,” meaning “a series of rapes, burglaries, arsons, or murders.” Ressler was so impressed by the phrase that, upon returning to Quantico, he began to use the term “serial killer” in his own lectures to describe “the killing of those who do one murder, then another and another in a fairly repetitive way.” However, there is a problem with this story, since there is a documented proof that this expression existed at least a dozen years before Ressler supposedly invented it. First documented use of this term is traced back to 1961, and is used in Merriam Webster’s 1961 Third New International Dictionary. However, it was Ressler who altered the phase from serial murderer to serial killer.

But there is no clear definition invented to define a serial killer however there is three important elements to describe a Serial Killer:

Firstly, the person should have at least killed three people. The place of the murders should all be different locations and lastly, time, there should be a "cooling off period" an interval between murders that could range from hours to years. However, the problem with this that it is too broad and overly narrow, but the people from the National Institutes of Justice formulated a more flexible and accurate definition by which the authorities regard as more suitable.

A series of two or more murders, committed as separate events, usually, but not always, by one offender acting alone. The crimes may occur over a period of time ranging from hours to years. Quite often the motive is psychological, and the offender’s behavior and the physical evidence observed at the crime scenes will reflect sadistic, sexual overtones.
-National Institutes of Justice

Recently, i acquired some knowledge about Killings way back in the 19th century and thought it would be a great example for this article. I found it at crime.about.com entitled Jack The Ripper by Charles Montaldo. You might have already know who Jack The Ripper is, if you don't let me tell you a bit about him; A serial killer murdered and mutilated at least five prostitutes in the East End of London in 1888, No one was ever arrested until today 128 years later, so now you know who he is, but  who is he actually?

"Jack The Ripper" is the name given to unknown killer due to correspondence at the time from someone claiming to be the killer signed with that name. But while the murders were taking place, the assailant was know as the Whitechapel Murder or "Leather Apron."

It is extremely impossible that anyone would uncover who Jack The Ripper is, even though people are still uncovering material that can help the case, the discovery of something unarguable has to be regarded as a long-shot.