1. <programming> 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. <communications, security> 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. <communications> cost control callback.