###Answer the question from the input provided. Please answer from input only.

###Input: Between about 1964 and 1973, large amounts of older material stored in the BBC's various video tape and film libraries were either destroyed,[note 3] wiped, or suffered from poor storage which led to severe deterioration from broadcast quality. This included many old episodes of Doctor Who, mostly stories featuring the first two Doctors: William Hartnell and Patrick Troughton. In all, 97 of 253 episodes produced during the first six years of the programme are not held in the BBC's archives (most notably seasons 3, 4, & 5, from which 79 episodes are missing). In 1972, almost all episodes then made were known to exist at the BBC, while by 1978 the practice of wiping tapes and destroying "spare" film copies had been brought to a stop.

Some episodes have been returned to the BBC from the archives of other countries who bought prints for broadcast, or by private individuals who acquired them by various means. Early colour videotape recordings made off-air by fans have also been retrieved, as well as excerpts filmed from the television screen onto 8 mm cine film and clips that were shown on other programmes. Audio versions of all of the lost episodes exist from home viewers who made tape recordings of the show. Short clips from every story with the exception of Marco Polo, "Mission to the Unknown" and The Massacre of St Bartholomew's Eve also exist.

"Official" reconstructions have also been released by the BBC on VHS, on MP3 CD-ROM, and as special features on DVD. The BBC, in conjunction with animation studio Cosgrove Hall, reconstructed the missing episodes 1 and 4 of The Invasion (1968), using remastered audio tracks and the comprehensive stage notes for the original filming, for the serial's DVD release in November 2006. The missing episodes of The Reign of Terror were animated by animation company Theta-Sigma, in collaboration with Big Finish, and became available for purchase in May 2013 through Amazon.com. Subsequent animations made in 2013 include The Tenth Planet, The Ice Warriors and The Moonbase.

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]

The serials The Deadly Assassin and Mawdryn Undead and the 1996 TV film would later establish that a Time Lord can only regenerate 12 times, for a total of 13 incarnations. This line became stuck in the public consciousness despite not often being repeated, and was recognised by producers of the show as a plot obstacle for when the show finally had to regenerate the Doctor a thirteenth time. The episode "The Time of the Doctor" depicted the Doctor acquiring a new cycle of regenerations, starting from the Twelfth Doctor, due to the Eleventh Doctor being the product of the Doctor's twelfth regeneration from his original set.



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

###Answer: {'answer':'regeneration'}