Cogeneration: a process that converts a fuel into both thermal and electrical energy. The thermal energy may be in the form of steam, hot water, or hot air, or any combination of the three. This term is becoming obsolete, and is gradually moving towards the "CHP" (combined heat and power) term. Also includes District Energy: "District energy systems produce steam, hot water or chilled water at a central plant and then pipe that energy out to buildings in the district."