The general-purpose heat source is a U.S. DOE-designed radioactive heat source for radioisotope thermoelectric generators (RTG) or Stirling radioisotope generators (SRG).
The multi-mission radioisotope thermoelectric generator (MMRTG) is a type of radioisotope thermoelectric generator developed for NASA space missions such as the Mars Science Laboratory (MSL), under the jurisdiction of the United States Department of Energy's Office of Space and Defense Power Systems within the Office of Nuclear Energy.
Thermoelectric generator – (also called thermogenerators) are devices which convert heat (temperature differences) directly into electrical energy, using a phenomenon called the "Seebeck effect" (or "thermoelectric effect").
GPHS-RTG or General Purpose Heat Source — Radioisotope Thermoelectric Generator, is a specific design of the Radioisotope Thermoelectric Generator (RTG) used on US space missions.