Victim: A person who has been killed or suffered physical or mental anguish or loss of property as a result of an actual or attempted criminal offense committed by another person.
From the Dictionary of Criminal Justice 4th ed by George E. Rush which can be found in the Reference section of the Library Call# REF HV 7411 R87 1994.
Encyclopedia of Victimology and Crime Prevention by 
Serial Murderers and Their Victims by SERIAL KILLER VICTIM SELECTION
"No one knows for sure why a serial killer will choose a certain individual as their victim. When asked why, serial killers often give a wide range of answers regarding the reasons for their murders. The most common belief is that the killer wants to feel complete control over another person. They thrive on the fear their victims display and see the murder as the ultimate form of dominance over a human being.
"In order for any person to be defined as a serial killer, they must satisfy a few criteria, specified by the Federal Bureau of Investigation. The person
in question must have murdered a minimum of three individuals (not simultaneously), there must be a period of time in between the murders (to prove that multiple victims were not killed during a single fit of rage), and the circumstances of each murder should indicate that the killer felt a sense of dominance over the people they have killed. The victims must also be vulnerable to the killer in some way, a characteristic which indicates that the killer has sought to achieve a feeling of superiority.
"Many experts agree that serial killers have a vision in mind of their victim. This person would be thought of as their “ideal victim” based on race, gender, physical characteristics, or some other specific quality. It is rarely possible for the killers to find people who meet these exact qualifications, so they generally seek out people with similar traits. This is why serial killings often seem to be completely random at first – each victim may have something in common that only the killer easily recognizes.
"It is generally accepted that most serial killers feel a strong urge to commit acts of murder. They are, however, thought to be extremely cautious people who will not choose a victim unless they feel the chances of success are very high. For this reason, the first murder victim is very often a prostitute or homeless person, someone the killers can attack without drawing a lot of attention. These factors make it even more difficult to establish patterns in a series of slayings and to track down the responsible culprit."
“Serial Killer Victim Selection.” (n.d.) Retrieved on April 23, 2015, from
http://www.crimemuseum.org/crime-library/serial-killer-victim-selection#