An individual murderer (as opposed to an organization) kills three or more people on separate occasions in service of abnormal psychological gratification, with a significant period of time taking place in between the murders.

Notes:
There is no widely accepted and precise definition for this category of crime. Prefer to omit this theme if it is unclear whether a serial murder was part of the story. Do not use this theme if the murders are primarily motivated by, for example, good old greed (not an abnormal psychological gratification).

References:
https://en.wikipedia.org/wiki/Serial_killer