The Collaborative International Dictionary of English v.0.48:

call-back \call-back\ n.
   1. a return call, especially a telephone call.
      [WordNet 1.5]

   2. the recall of an employee to work after a layoff.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

callback
    n 1: a request by the manufacturer of a defective product to
         return the product (as for replacement or repair) [syn:
         recall, callback]

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

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 PABXs 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)