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The Chameleon Arch was a piece of Time Lord equipment which could modify the biology of a creature so the cells were that of something else. 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 (although the plan eventually fails). (DW: Human Nature)
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. (DW: Utopia)
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 Doctor Who: The Movie)
The Doctor also briefly considered that Jackson Lake might be a future incarnation of himself under the influence of an Arch. (DW: The Next Doctor)