The Believer' is a term used to describe the character Fox Mulder from the popular television show 'The X-Files'. Mulder, played by David Duchovny, is an FBI agent who investigates unexplained phenomena and paranormal activity. He is known for his strong belief in the existence of extraterrestrial life and other paranormal events. The term 'The Believer' is often used to describe Mulder's unwavering conviction in the face of skepticism and opposition.