This list contains terms related to geothermal energy and technologies.

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Natural condition of the environment at any given time.


Water-bearing stratum of permeable sand, rock, or gravel.

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Baseload Plants

Electricity-generating units that are operated to meet the constant or minimum load on the system. The cost of energy from such units is usually the lowest available per unit of electricity. 

Binary-Cycle Power Plant

A geothermal electricity generating plant using heat from lower temperature reservoirs. The technology uses the heat of the geothermal fluid (the "primary fluid") to vaporize  a “working fluid” with a lower boiling point, which drives a turbine/generator set to generate electricity. Learn more about electricity generation in geothermal power plants.


Subsurface fluids containing appreciable amounts of sodium chloride or other salts, from which lithium can be extracted.

British Thermal Unit (Btu)

The quantity of heat required to raise the temperature of one pound of water one degree Fahrenheit at standard conditions. (Equal to 252 calories.)

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Cap Rocks

Rocks of low permeability that overlie a geothermal reservoir.

Capacity Factor

The ratio of actual energy output to possible energy output. Among renewable power sources, geothermal energy delivers the highest capacity factor. 

Cascading Heat

A process that uses a stream of geothermal hot water or steam to perform successive tasks requiring lower and lower temperatures.


Creating geothermal energy from oil and gas wells that are still active. Oil and gas wells often encounter hot or warm fluids along with oil and gas, and co-production captures the heat from this hot or warm fluid to generate electricity, which can be used immediately or stored for later use.


Water formed by condensation of steam.


Equipment that condenses turbine exhaust steam into condensate.

Cooling Tower

A structure in which heat is removed from hot water formed by condensation of steam.


Earth's outer layer of rock. Also called the lithosphere.

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Direct Use

Use of geothermal heat without first converting it to electricity, such as for space heating and cooling, food preparation, industrial processes, etc.


Boring into the Earth to access geothermal resources, usually with oil and gas drilling equipment that has been modified to meet geothermal requirements.

Dry Steam

Very hot steam that doesn't occur with liquid. Dry steam power plants use hydrothermal fluids that are already mostly steam, which is a relatively rare natural occurrence. Dry steam power plant systems are the oldest type of geothermal power plants, and were first used in Lardarello, Italy, in 1904. Learn more about electricity generation from geothermal power plants.

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The ratio of the useful energy output to energy input of a machine or other energy-converting plant.

Enhanced Geothermal Systems

Human-made underground geothermal reservoirs that extract geothermal energy from the Earth for electricity generation and/or heating applications. EGS reservoirs are created by drilling wells and injecting water to stimulate the rocks and create permeability. 

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A fracture or fracture zone in the Earth's crust along which slippage of adjacent Earth material occurs.

Flash Steam

Steam produced when the pressure on a geothermal liquid is reduced. The depressurization of geothermal fluid to transform it into steam is also called “flashing”. Flash steam plants are the most common type of geothermal power plants in operation today. Learn more about electricity generation from geothermal power plants.


A vent or hole in the Earth's surface, usually in a volcanic region, from which steam, gaseous vapors, or hot gases issue.

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Study of the planet Earth—its composition, structure, natural processes, and history.


Also known as downhole sensors, these collect high-resolution, real-time micro-seismic data to inform how a geothermal reservoir is growing. Geothermal developers need this information to adjust operations and design the most effective and efficient EGS reservoirs.


Of or relating to the Earth's interior heat.

Geothermal District Heating (GDH) 

A type of direct use in which a utility system supplies multiple users with hot water or steam from a central plant or well field through a distribution network.

Geothermal Energy

The Earth's interior heat made available to humanity by extracting it from hot water or rocks. This is literally the “heat beneath our feet,” or heat that flows continuously from the Earth’s interior to the surface. This heat has been radiating from the earth’s core for about 4.5 billion years. 

Geothermal Gradient

The rate of temperature increase in the Earth as a function of depth. Temperature increases an average of 1° Fahrenheit for every 75 feet in descent.

Geothermal Heat Pumps

Mature technologies that use the constant temperature of the shallow earth  (40–70°F) to provide heating and cooling solutions to buildings wherever the ground can be cost-effectively accessed to depths below seasonal temperature variations.

Geothermal heat pumps increase the efficiency and reduce the energy consumption of heating and cooling systems in residential and commercial buildings. They are currently deployed across all 50 states, and the market is growing as their value becomes better understood.


A spring that shoots jets of hot water and steam into the air. 

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Heat Exchanger

A device for transferring thermal energy from one fluid to another.

Heat Flow

Movement of heat from within the Earth to the surface, where it is dissipated into the atmosphere, surface water, and space by radiation.

Hydrothermal Resource

A conventional geothermal resource (Underground system of hot water and/or steam) that can be tapped for electricity generation using existing technologies.

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The process of returning spent geothermal fluids to the subsurface. Sometimes referred to as reinjection.

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Kilowatt (kW)

1,000 watts—a unit of electric power.

Kilowatt-Hour (kWh)

The energy represented by 1 kilowatt of power consumed for a period of 1 hour, equal to 3,413 Btus. 

Known Geothermal Resource Area (KGRA)

A region identified by the U.S. Geological Survey as containing geothermal resources.

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A soft, light metal, found in rocks and subsurface fluids called brines. As a critical mineral, lithium is crucial to domestic high-tech manufacturing, especially for electric vehicle and grid storage batteries and components. Learn more in the Lithium StoryMap.


The simultaneous demand of all customers required at any specified point in an electric power system.

Low Temperature Resources

Low-temperature geothermal resources are generally considered those below 300°F (150°C), which are ubiquitous in shallow soil, rock, and/or aquifers and have valuable thermal storage properties. Learn more about GTO’s work in this area.

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Molten rock within the Earth, from which igneous rock is formed by cooling.

Machine Learning

The use of advanced algorithms to identify patterns and make inferences from data. Machine learning and artificial intelligence can provide insights on large, complicated datasets, including those used to analyze geothermal energy. The rapidly advancing field of machine learning offers substantial opportunities for technology advancement and cost reduction throughout the geothermal project lifecycle, from resource exploration to power plant operations. 


The Earth's inner layer of molten rock, lying beneath the Earth's crust and above the Earth's core of liquid iron and nickel.

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Peaking Plants

Electricity generating plants that are operated to meet the peak or maximum load on the system. The cost of energy from such plants is usually higher than from baseload plants.


The capacity of a solid substance (such as rock) to transmit a fluid. The degree of permeability depends on the number, size, and shape of the pores and/or fractures in the rock and their interconnections. It is measured by the time it takes a fluid of standard viscosity to move a given distance. The unit of permeability is the Darcy.

Plate Tectonics

A theory of global-scale dynamics involving the movement of many rigid plates of the Earth's crust. Tectonic activity is evident along the margins of the plates where buckling, grinding, faulting, and vulcanism occur as the plates are propelled by the forces of deep-seated mantle convection currents. Geothermal resources are often associated with tectonic activity, since it allows groundwater to come in contact with deep subsurface heat sources.


The ratio of the aggregate volume of pore spaces in rock or soil to its total volume, usually stated as a percent. 

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A natural underground container of liquids or gases, such as water or steam (or, in the petroleum context, oil or gas).

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A measure of the quantity or concentration of dissolved salts in water.


A sinking of an area of the Earth's crust due to fluid withdrawal and subsurface pressure decline.

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Total dissolved solids (TDS)

Term used to describe the amount of solid materials in water.

Thermal Gradient

The rate of increase or decrease in the Earth's temperature relative to depth.

Transmission Line

Structures and conductors that carry bulk supplies of electrical energy from power-generating units.


A bladed, rotating engine activated by the reaction or impulse, or both, of a directed current of fluid. In electric power applications, such as geothermal plants, the turbine is attached to and spins a generator to produce electricity.

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A geothermal reservoir system in which subsurface pressures are controlled by vapor rather than by liquid. Sometimes referred to as a dry-steam reservoir.

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Well Logging

Assessing the geologic, engineering, and physical properties and characteristics of geothermal reservoirs with instruments placed in the wellbore.

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