The segment at the beginning of an EDI transaction that specifies where the EDI message starts.

Merriam-Webster Online Dictionary
header (noun)
one that removes - heads , especially a grain-harvesting machine that cuts off the grain heads and elevates them to a wagon
a) a brick or stone laid in a wall with its end toward the face of the wall
b) a beam fitted at one side of an opening to support free ends of floor joists, studs, or rafters
c) a horizontal structural or finish piece over an opening - lintel
d) a conduit (as an exhaust pipe for a many-cylindered engine) into which a number of smaller conduits open
e) a mounting plate through which electrical terminals pass from a sealed device (as a transistor)
a fall or dive - headfirst took a header into the pool
a shot or pass in soccer made by the ball - heading
- head