Mimic device
From The Stargate Omnipedia
Alien device used by the mimetic aliens. The device allows a complete physical duplication of any individual that has been processed.
KEY EPISODES
Foothold - Aliens overtake the S.G.C. and use mimic devices to masquerade as Stargate personnel.
Smoke and Mirrors - Twelve mimic devices are stolen from Area 51 and used to further the plans of the N.I.D.