Source : WordNet®

callback
     n : a request by the manufacturer of a defective product to
         return the product (as for replacement or repair) [syn: {recall}]

Source : Free On-Line Dictionary of Computing

callback
     
        1.  A scheme used in {event-driven} programs
        where the program registers a {subroutine} (a "callback
        handler") to handle a certain {event}.  The program does not
        call the handler directly but when the event occurs, the
        {run-time system} calls the handler, usually passing it
        arguments to describe the event.
     
        2.  A {user authentication} scheme
        used by some computers running {dial-up} services.  The user
        dials in to the computer and gives his {user name} and
        {password}.  The computer then hangs up the connection and
        uses an {auto-dial} {modem} to call back to the user's
        registered telephone number.  Thus, if an unauthorised person
        discovers a user's password, the callback will go, not to him,
        but to the owner of that login who will then know that his
        account is under attack.
     
        However, some {PABX}s can be fooled into thinking that the
        caller has hung up by sending them a dial tone.  When the
        computer tries to call out on the same line it is not actually
        dialing through to the authorised user but is still connected
        to the original caller.
     
        3.  {cost control callback}.
     
        (2003-07-13)