Shimmer

A Shimmer was a cloaking device used by the Vinvocci to change their appearance. These devices, disguised as a wristwatch, allowed Rossiter and Addams to work for Joshua Naismith undetected. Apparently the device is uncomfortable to use and the name of the device implies that the cloaking process involves light. The Doctor could easily detect a shimmer even when not looking at it when he deactivaed Rossiter's one. (DW: The End of Time)