Source : Webster's Revised Unabridged Dictionary (1913)

Header \Head"er\, n.
   1. One who, or that which, heads nails, rivets, etc., esp. a
      machine for heading.

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

   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.

   4. A reaper for wheat, that cuts off the heads only.

   5. A fall or plunge headforemost, as while riding a bicycle,
      or in bathing; as, to take a header. [Colloq.]

Source : WordNet®

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}, {head}]
     2: horizontal beam used as a finishing piece over a door or
        window [syn: {lintel}]
     3: brick that is laid sideways at the top of a wall [syn: {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"

Source : Free On-Line Dictionary of Computing

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.