Source : Webster's Revised Unabridged Dictionary (1913)

Overhead \O`ver*head"\, adv.
   Aloft; above; in or attached to the ceiling or roof; in the
   story or upon the floor above; in the zenith.

         While overhead the moon Sits arbitress.  --Milton.

   Note: Also used adjectively; as, an overhead crane, gear,
         etc.

   {Overhead engine}, a vertical steam engine in which the
      cylinder stands above the crank.

   {Overhead work}, a general term in manufactories for
      countershafting and gearing, when overhead.

Source : WordNet®

overhead
     adj : located or originating from above; "an overhead crossing"
           [ant: {surface}, {subsurface}]
     n 1: the expense of maintaining property (e.g., paying property
          taxes and utilities and insurance); it does not include
          depreciation or the cost of financing or income taxes
          [syn: {operating expense}, {operating cost}, {budget
          items}]
     2: (computer science) the processing time required by a device
        prior to the execution of a command [syn: {command
        processing overhead time}, {command processing overhead},
        {command overhead}]
     3: (computer science) the disk space required for non-data
        information (used for location and timing) [syn: {disk
        overhead}]
     4: a transparency for use with an overhead projector [syn: {viewgraph}]
     5: (nautical) the top surface of an enclosed space on a ship
     6: a hard return hitting the tennis ball above your head [syn:
        {smash}]

overhead
     adv 1: above your head; in the sky; "planes were flying overhead"
     2: above the head; over the head; "bring the legs together
        overhead"

Source : Free On-Line Dictionary of Computing

overhead
     
        1. Resources (in computing usually processing time or storage
        space) consumed for purposes which are incidental to, but
        necessary to, the main one.  Overheads are usually
        quantifiable "costs" of some kind.
     
        Examples: The overheads in running a business include the cost
        of heating the building.  Keeping a program running all the
        time eliminates the overhead of loading and initialising it
        for each transaction.  Turning a {subroutine} into {inline}
        code eliminates the call and return time overhead for each
        execution but introduces space overheads.
     
        2.  information, such as control, routing, and
        error checking characters, that is transmitted along with the
        user data.  It also includes information such as network
        status or operational instructions, network routing
        information, and retransmissions of user data received in
        error.
     
        3. Overhead transparencies or "slides" (usually 8-1/2" x 11")
        that are projected to an audience via an overhead (flatbed)
        projector.
     
        (1997-09-01)