Mark III nuclear warheads are a type of weapon used by the Tau'ri.
A Mark III warhead was sent on the first original trip through the Stargate to the planet Abydos, to be used in case the leader of the expedition, Colonel Jack O'Neill, detected any threat that could reach Earth. In his mind, O'Neil was going to send his team back to Earth, while he stayed behind and committed suicide by detonating the bomb.
However, the Supreme System Lord Ra's soldiers found and took the bomb. Ra intended to send it back to Earth, with the quartzite-like mineral, Naquadah, included, which would increase it's destructive power a hundredfold, thus producing cataclysmic results that would destroy Earth. But then, after Ra was leaving the planet due to the rebellion, O'Neill and Dr. Daniel Jackson sent it on Ra's mothership via transportation rings a few seconds before detonation, obliterating Ra and his ship. (Stargate)