Vocuum
From The Stargate Omnipedia
Small, cylindrical Goa'uld device designed to project an enormous three-dimensional image, generally of a Goa'uld commander or System Lord. It is typically used to summon troops, give orders, authorize spokespersons, or dispensing other messages to battalions of Jaffa while the commander is away.
Use of the vocuum during battle operations is strictly forbidden in the Rules of Engagement.
KEY EPISODE
Rules of Engagement - Carter successfully merges the vocuum with Earth-based technology, allowing a playback of the dying Apophis to be displayed to his unsuspecting human troops.