A victim of a particular form of Goa'uld mind control, who remains totally unaware that he or she has been programmed with a complex layer of false memories to cover up the brief time it takes to abduct and program him or her. The affected individual is susceptible to a specific trigger, be it a bit of dialogue or an event.
When this happens, the program takes over and the za'tarc sets out on his or her mission -- armed with a powerful energy weapon disguised as a ring. If prevented from completing the objective, they will terminate themselves. Pressing on the center of the weapon causes it to build up a charge and explode, killing the wearer.
The Tok'ra possess a piece of technology that is theoretically capable of detecting whether someone has become a victim of this technology. Working in conjunction with the Tok'ra memory device, the za'tarc detector compares the conscious mind to the subconscious. If they do not match, it is concluded that the person's memories have been altered without their knowledge.
Divide and Conquer - After the Tok'ra and S.G.C. use a za'tarc detector on all their people, Martouf is discovered to be a za'tarc, programmed to kill the President of the United States.