The Collaborative International Dictionary of English v.0.48:

Header \Head"er\ (h[e^]d"[~e]r), n.
   1. One who, or that which, heads nails, rivets, etc., esp. a
      machine for heading.
      [1913 Webster]

   2. One who heads a movement, a party, or a mob; head; chief;
      leader. [R.]
      [1913 Webster]

   3. (Arch.)
      (a) A brick or stone laid with its shorter face or head in
          the surface of the wall.
      (b) In framing, the piece of timber fitted between two
          trimmers, and supported by them, and carrying the ends
          of the tailpieces.
          [1913 Webster]

   4. A reaper for wheat, that cuts off the heads only.
      [1913 Webster]

   5. A fall or plunge head first, as while riding a bicycle, or
      a skateboard, or in bathing; -- sometimes, implying the
      striking of the head on the ground; as, to take a header.
      [Colloq.]
      [PJC] Headfirst

WordNet (r) 3.0 (2006):

header
    n 1: a line of text serving to indicate what the passage below
         it is about; "the heading seemed to have little to do with
         the text" [syn: heading, header, head]
    2: horizontal beam used as a finishing piece over a door or
       window [syn: header, lintel]
    3: brick that is laid sideways at the top of a wall [syn:
       header, coping, cope]
    4: a framing member crossing and supporting the ends of joists,
       studs, or rafters so as to transfer their weight to parallel
       joists, studs, or rafters
    5: a machine that cuts the heads off grain and moves them into a
       wagon
    6: (soccer) the act of hitting the ball with your head
    7: a headlong jump (or fall); "he took a header into the
       shrubbery"

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "header":
   belly buster, belly flop, belly whopper, cannonball, crash dive,
   cropper, culbute, dive, drop, fall, flop, forced landing, gainer,
   jackknife, nose dive, parachute jump, pitch, plunge, pounce,
   power dive, pratfall, running dive, sky dive, spill, sprawl,
   stationary dive, stoop, stumble, swan dive, swoop, trip, tumble



The Free On-line Dictionary of Computing (30 December 2018):

header

   1. The portion of a packet, preceding the actual data,
   containing source and destination addresses, error checking
   and other fields.

   2. The part of an electronic mail message or news article
   that precedes the body of a message and contains, among other
   things, the sender's name and e-mail address and the date and
   time the message was sent.