(n) in airline jargon, a pilot scheduled to ride along or presently on-board the cockpit of a plane flown by another pilot
(v) the act of such a pilot undertaking such a flight
Such pilots are so-called, possibly, because they're often qualified to head up the flights yet serve no official function. The term was made more widely known by Frank W. Abagnale Junior in his autobiography, Catch Me If You Can (along with major film and stage adaptations).
"Are you my deadhead to Miami?" -a smiling Stewardess in the film