Serial murder has been a subject of extensive criminological, psychological, and sociological inquiry. Despite the long-standing interest in the phenomenon, defining serial murder remains contentious among scholars and law enforcement professionals. The lack of consensus on a standard definition has implications for research and investigation. This chapter explores various definitions of serial murder, examining historical perspectives, contemporary research interpretations, and frameworks to propose a comprehensive understanding of the term. Additionally, we briefly consider the prevalence of serial homicide and clarify the distinctions between serial, spree, and mass killers to orient the reader to key categories of multiple homicide offenders.