You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Doctor_Who

Context:
Producers introduced the concept of regeneration to permit the recasting of the main character. This was first prompted by original star William Hartnell's poor health. The actual term "regeneration" was not initially conceived of until the Doctor's third on-screen regeneration however; Hartnell's Doctor had merely described undergoing a "renewal," and the Second Doctor underwent a "change of appearance".[citation needed] The device has allowed for the recasting of the actor various times in the show's history, as well as the depiction of alternative Doctors either from the Doctor's relative past or future.[citation needed]

Question:
What term is used to explain a change in the appearance of Doctor Who?

regeneration