A shimmer was a cloaking device used by the Vinvocci to change their appearance.
Appearance and function Edit
These devices were disguised as wristwatches and, while effective, were apparently uncomfortable to use. The Tenth Doctor could easily detect a shimmer even when not looking directly at it, as he did when he deactivated Rossiter's shimmer. (TV: The End of Time)
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)