Chameleon Arch

The Chameleon Arch was a piece of Time Lord technology, which could modify the biology of a Gallifreyan so the cells registered as another species. The procedure was very painful. It was composed of a headset and a fob watch-like device, with an added perception filter, used to store the Gallifreyan biological information.

The Chameleon Arch changes every cell in the user's body. However some traits of the user's true Time Lord heritage may remain; John Smith was able to deliberately cause a complex chain reaction to save a mother and her baby from being crushed by a piano. Smith also kept a dream journal in which he chronicled memories of the Doctor's adventures, while Professor Yana heard drums beating in his head all his life, and both Smith and Yana possessed great intellects.

The Doctor used one to change himself into the Human John Smith so he wouldn't have to punish the Family of Blood. 

The Master also used it to turn into a Human to escape the Last Great Time War, and remained in Human form until he opened the watch and remembered everything that the Doctor had done to him. There is a possibly apocryphal account suggesting the Seventh Doctor, prior to his regeneration into his eighth incarnation, made use of an Arch to create the fiction that he was half-human, to use against The Master (IDW: The Forgotten, referencing events depicted in )

It was also used to help the Doctor recover his memories, after his mind was taken over by Es'Cartress of the Tactire. (IDW: The Forgotten)

The Doctor also briefly considered that Jackson Lake might be a future incarnation of himself under the influence of an Arch. Lake did possess what turned out to be just an ordinary fob watch; his initials at the bottom revealed his identity to himself and the Doctor. 

There are only two known chameleon arches left in the universe, one that once belong to the Doctor in his TARDIS before he gave it to it Timothy Latimer, and one that was in the possession of the Master before he left the end of the universe to the 21st century.
 * It is unknown what happened to the Master's fob watch, after he opened it

Behind the scenes

 * As noted above, the introduction of the Chameleon Arch has been used in some spin-off media to help rectify some continuity issues, most notably the "half-human" scenario of the 1996 TV movie. This can also be extended to potentially allow other spin-off productions, such as The Stranger and The Airzone Solution to be rationalised within continuity. To date, however, the TV series itself has yet to utilise the Arch for any such purpose.