What is Dexter's internal urge to kill nicknamed in the series?

Dexter’s internal urge to kill is nicknamed the “Dark Passenger.”

The phrase describes the obsessive, psychologically driven force Dexter experiences as an inner presence urging him toward murder. He often imagines it as something separate from himself, allowing him to distance his everyday identity from his violent impulses.

The term originated in Jeff Lindsay’s Dexter novels and became especially prominent in the Showtime television series. In the show, Dexter connects the Dark Passenger to his childhood trauma, bloodlust, and compulsive need to kill, while Harry Morgan’s code gives that urge a set of rules and targets.

A common mix-up is treating the Dark Passenger as a literal supernatural being. The television series generally presents it as a metaphor for Dexter’s psychology, unlike some of the novels, which explore more supernatural possibilities. Later episodes also question whether the Passenger is truly separate from Dexter or simply a name he gives his own choices.