A shimmer was a cloaking device that allowed its owner to change their appearance. The Eleventh Doctor once described it as "an all-in-one disguise kit for aliens that don't look human". (TV: The End of Time, COMIC: Ripper's Curse)
Appearance and function
Shimmer devices used by Vinvocci were disguised as wristwatches and, while effective, were apparently uncomfortable to use. The Tenth Doctor could easily detect such a shimmer even when not looking directly at it, as he did when he deactivated Rossiter's shimmer. (TV: The End of Time)
History
Mac'atyde, better known as serial killer Jack the Ripper, used a shimmer suit to disguise himself as human and lure his victims. (COMIC: Ripper's Curse)
Rossiter and Addams picked up a signal from a piece of medical equipment and used shimmers to work undetected for Joshua Naismith. The Doctor deactivated them with his sonic screwdriver. (TV: The End of Time)