Numismaton gas was an extremely rate gas, a supply of which existed at the core the planet Sarn.
The gas had "infinite transformative powers"; according to the Master, it was one of the most powerful things in the universe. When ignited, it could heal wounds and other afflictions. The Fifth Doctor knew off-hand of nusmimaton gas as a substance that could help a Time Lord survive after they'd exhausted their regeneration cycle.
After accidentally shrinking himself with an experimental Tissue Compression Eliminator, the Master used the flame to restore himself to his proper size. (TV: Planet of Fire) He also hoped to gain an actual new regeneration cycle from the ordeal; although the Celestial Intervention Agency temporarily believed he had succeeded when he managed to survive severe burns immediately after his use of the flame, (PROSE: CIA File Extracts) he had actually survived thanks to the gas boosting the lingering power of the Source of Traken inside him. (PROSE: The Quantum Archangel)
Though he survived, he was wounded, requiring further supernatural healing to fully restore himself; (PROSE: A Town Called Eternity) by the time of his confrontations with the Seventh Doctor on the Cheetah Planet, (TV: Survival) the same incarnation of the Master still had no regenerations left, thus proving the CIA's assumption wrong. (PROSE: First Frontier, The Eight Doctors, etc.)