The best replacement for callback is postback.

The best alternatives to callback are rated as following:
★★★★★  postback
★★★★  event
★★★★  promise
★★★  closure
★★★  delegate
★★★  call back
★★  listener
★★  event c#
★  function pointer
★  polling
★  partial postback
★  observer
★  updatepanel
  observer pattern
  callout
  event handler
  handler

Your alternative for callback is not listed? Suggest!
Speaking about callback, one probably shouldn't forget about ajax.

Other search terms related to callback and their ratings are:
★★★★★ ajax
★ anonymous function
You have in mind a companion for callback and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of callback in all years. Click on graph for more.
callback
postback
event