The TER, or transphase eradication rod, is Goa'uld technology created to defeat the Reetou, alien creatures that are invisible in our dimension. It relies on the Reetou emissions to which the Goa'uld are sensitive, in order to make the Reetou visible in our phase and to terminate them. The Tok'ra have acquired the technology, and Jacob brought several TERs to the SGC to successfully defeat the Reetou rebels that had infiltrated the base. The TER also compensates for phase-shifting that is characteristic of Goa'uld invisibility devices, and it has been used successfully against the personal cloaking technology used by Nirrti.
Cross Reference: Alternate Dimension, Invisibility, Nirrti, Reetou, Weapons
Episode Reference: Show and Tell, Fair Game, Rite of Passage