Defensive weapon employed by the Goa'uld and Tok'ra in the sci-fi television series Stargate SG-1.
"Tacs," as they are commonly called, are small black spheres (slightly larger than a softball) that fire energy blasts from a red iris housed in the front of the device. These automatic, heat-seeking weapons and are generally placed around an entrance (such as doorway) to create a defensive perimeter, keeping would-be attackers at bay.
Tacs appeared first in episode 307, Dead Man's Switch, when the Tok'ra fugitive Korra used them to defend the cave in which he was hiding from the alien bounty hunter Aris Boch.
Though Tac blasts don't appear to be amazingly powerful in and of themselves—both zatnicketels and staff weapons seem to pack more of a punch—they fire fairly rapidly and thus a few well positioned Tacs can cover a small area with fire very quickly, creating quite an effective ambush.
Like most technology in the SG-1 universe, Tacs can be disabled by a zatnicketel discharge, or just by beating the hell out of them with a rock—depending on what is handy at the time. (Just fyi, in case you ever run across one in real life.)
See also these other fine examples of Stargate SG-1 weaponry: staff weapon, zatnicketel, intar