The sign of shol'va. From the Stargate SG-1 Season 1 episode, "Bloodlines".
Shol'va is the Goa'uld word for traitor. It refers to a Jaffa who has betrayed the Goa'uld to whom he was in service, with the main example being Teal'c of Chulak. Teal'c was branded a "shol'va" when he defected from the service of his master Apophis and joined the Tau'ri and Stargate Command in order to help his fellow Jaffa gain freedom from their Goa'uld masters. Many Goa'uld and even some Jaffa who are loyal to the Goa'uld call Teal'c "shol'va" instead of his first name.
The word also has heretical connotations, considering that the Goa'uld are seen as the Jaffa's gods. (SG1: "The Nox", "Bloodlines")
Teal'c himself uses the term, with all its venom and contempt, of Bra'tac when the younger Jaffa is once more under the sway of Apophis. A related term appears to be "ha'shak", although this seems to refer to betrayal of the individual, rather than the god. (SG1: "Threshold")