A "dead head" is a crewmember who is assigned to fly as a passenger on a specific flight (not working it) so they can get to another city to work where they will pick up their assigned trip sequence.
This is different from a "Non-Rev" as that is a term used for people using airine benefits to travel for their own needs, not at the companys direction. A dead heading crewmember can bump a revenue passenger off a flight, while a non-rev cannot.
BTW- Dead heads are getting paid to be on the flight, as it is a work assignment, even though they are sitting in back.
Regarding this answer from below by Aeirol 1970-
"DEADHEAD MEANS,BASICALLY When you work for the airline being pilot,and or flight attendant,you have ended your trip in one state but you live in other.so you catch the next flight home."
UHHHH NO. It's not the same! That is called COMMUTING. If it were a deadhead, the company would require you to be on the flight. Getting to & from work is on your own! I spent 7 hours getting bumped off flights getting home from work tonight while commuting. I wish I were a "deadhead" as I would have gotten on a flight! (Had to drive.)