Regeneration

From TARDIS Information System, the free Doctor Who reference database:

Regeneration is the process by which the Doctor and other Time Lords maintain their centuries-long lifespans. It is generally described as a "genetic reshuffling" which a Time Lord may go through if his body is worn out by age or has suffered some life-threatening injury or infection. Time Lords may even regenerate at will.

While the Doctor apparently has never had much choice in what the finished product looks like, others, like Romana, seem adept enough at the process to custom design their new appearances, as demonstrated in the opening scene of "Destiny of the Daleks," when she went through several bodies before finally deciding on a copy of Princess Astra.

Regeneration was first introduced when the First Doctor (William Hartnell) changed into the Second Doctor (Patrick Troughton) in the closing moments of "The Tenth Planet." Tradition has it that Hartnell himself thought up the idea of the Doctor being able to change into a new body as a means of keeping the series going after his departure. The process was not actually called "regeneration" until the end of "Planet of Spiders," when the Third Doctor (Jon Pertwee) changed into the Fourth (Tom Baker). Prior to that, the Doctor was simply described as having "changed his appearance." In "Power of the Daleks" (the first story featuring the Second Doctor), the Doctor described himself as having been "renewed", and also said that the change was "part of the TARDIS."