The following definitions reflect those used by the International Energy Agency (IEA); definitions used by other organisations and publications may vary.


underground coal gasification


ultra-low CO2 steelmaking


International Union of Producers and Distributors of Electrical Energy




ultra-supercritical steam cycle

unconventional gas

sources of gas trapped deep underground by impermeable rocks, such as coal, sandstone and shale. The three main types of "unconventional" gas are: shale gas (found in shale deposits); coalbed methane, or CBM (extracted from coal beds) and tight gas (which is trapped underground in impermeable rock formations). While different techniques are applied, depending on the type of gas being extracted, one common method is known as hydraulic fracturing: large volumes of water (mixed with some sand and chemicals) are injected underground to create cracks in the rock, freeing the trapped gas so it can flow into the well bore created by the drill and be collected. Another key technology is horizontal drilling which enables the exposure of significantly more surface to the well.

unconventional oil

includes oil shale, oil sands-based extra heavy oil and bitumen, derivatives such as synthetic crude products, and liquids derived from natural gas – gas-to-liquid (GTL) or coal-to-liquid (CTL).