The best replacement for observer is listener.

The best alternatives to observer are rated as following:
★★★★★  listener
★★★★  callback
★★★★  observable
★★★  fel imp
★★★  mediator
★★  participant
★★  mediator pattern
★★  pubsub
★★  publish subscribe
★  command pattern
★  visitor pattern
★  shivarra
★  felhunter
★  imp
  observer
  complete participant
  x lander
  observant
  participant observer
  observer as participant
  actor

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Google Trends

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