call back vs callback

call back

noun
  • Request for a second interview or audition, following a successful initial interview. 

verb
  • To request someone's return. 

  • To telephone someone again at a more convenient time. 

  • To return to a shop or residence one has previously visited. 

callback

noun
  • A follow-up audition in casting. 

  • A joke which references an earlier joke in the same routine. 

  • The return of a situation to a previous position or state. 

  • A function pointer passed as an argument to another function. 

  • A return telephone or radio call; especially one made automatically to authenticate a logon to a computer network. 

  • A form of audience participation in which the audience shout lines in response to the dialog of a film. 

  • A product recall because of a defect or safety concern. 

How often have the words call back and callback occurred in a corpus of books? (source: Google Ngram Viewer )