A suicide implant is a device implanted into an individual's body. The device is designed to cause rapid death when activated.
In the Dominion, suicide implants were placed in the brain stems of all Vorta. Vorta who were captured were expected to commit suicide in this manner. The Vorta Keevan had been captured, and failed to commit suicide, and was expecting to be severely punished when returned to the Dominion. (DS9 episode: "The Magnificent Ferengi") Vorta could also be ordered to activate their suicide implant when ordered to do so by a superior officer or a Founder. Weyoun 6 activated his implant to save the life of Odo, and died in Odo's arms, happy that he had received Odo's blessing right before the end. (DS9 episode: "Treachery, Faith, and the Great River") Later, a frustrated Female Changeling stated that if the cloning facilities were available, she would order Weyoun 8 to activate his implant so he could be replaced. (DS9 episode: "Tacking Into the Wind")
Section 31 operatives also had a suicide implant embedded in their brains. They would activate the impant in the event that they were captured and facing interrogation. This device worked by depolarizing the nervous system of an individual. Luther Sloan used the device to prevent Julian Bashir and Miles O'Brien from extracting the cure to the morphogenic virus that Section 31 designed to commit genocide against the Founders. (DS9 episode: "Extreme Measures")