Final answer:
The FBI defines a serial killer as someone who commits at least three murders over a period of time with a cooling-off period between the murders.
Step-by-step explanation:
According to the Federal Bureau of Investigation (FBI), an individual must commit at least three murders over a period of time with a cooling-off period between the murders to be labeled a serial killer. This definition has been used by the FBI to differentiate a serial killer from other types of multiple murderers, such as mass murderers or spree killers, who may kill multiple victims in a shorter time span without the characteristic cooling-off period.