Refer to https://stackapps.com/questions/3740/are-api-key-and-app-key-the-same-thing. 

Although I've now retagged the questions that used [tag:app-key], there are [still plenty of places][1] where the [redundant term is used][2].  
The terms are interchangeable here, but we only need one tag, so please make [tag:app-key] a synonym of [tag:api-key].


  [1]: https://stackapps.com/search?tab=newest&q=%22app%20key%22
  [2]: https://stackapps.com/search?q=%22app%20keys%22