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Greening EPA Glossary

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Acid Rain: The result of sulfur dioxide (SO2) and nitrogen oxides (NOx) reacting in the atmosphere with water and returning to earth as rain, fog, or snow. For more information, visit EPA's Acid Rain website.

Air Handling Unit: A type of heating and/or cooling distribution that channels warm or cool air to different parts of a building. Equipment includes a blower or fan, heating and/or cooling coils, and related equipment such as controls, condensate drain pans, and air filters. Does not include ductwork, registers, grilles, boilers or chillers.

Air Side Economizer: An air economizer cycle reduces the load on the chilled water system by increasing the flow of outside air above the minimum required for ventilation when the outside air temperature is favorable in comparison to return air temperature. Also known as free cooling. For example, in cooling season, if there is cool, dry air outside, it will be drawn into the building to cool it, instead of using chillers to provide cooling.

Autoclave: A device that uses steam to sterilize equipment and deactivate bacteria, viruses, fungi, and spores.


Big Basin: The first approach at stormwater management, still widely used in the U.S., which involved large retention or detention basins or ponds. The basins detain and slow stormwater, allowing sediment, chemicals, and trash to be filtered out before the water is released into receiving waters. By reducing the velocity of water and controlling discharge rates, ponds reduce the likelihood of flooding and help to reduce the impact that impervious surfaces and development can have on water quality and aquatic habitats. However, these systems are not ideal since they do not manage the stormwater where it falls, often preventing infiltration and groundwater recharge and taking away wildlife habitat and available space for recreation or other site design needs.

Boiler Feed Water: Input water utilized by a boiler.

Building Automation System (BAS): A system that optimizes the start-up and performance of HVAC equipment and alarm systems. A BAS greatly increases the interaction between the mechanical subsystems of a building, improves occupant comfort, lowers energy use, and allows off-site building control.


Carbon Dioxide (CO2): A colorless, odorless, non-poisonous gas that exists in trace quantities (less than 400 parts per million) within ambient air. Carbon dioxide is a product of fossil fuel combustion. Although carbon dioxide does not directly impair human health, it is a greenhouse gas that traps terrestrial (i.e., infrared) radiation and contributes to the potential for global warming.

Carbon Dioxide Equivalent (CO2e): Emissions of greenhouse gases are typically expressed in a common metric so that their impacts can be directly compared, as some gases are more potent (i.e., have a higher global warming potential) than others. The international standard practice is to express greenhouse gases in carbon dioxide equivalents (CO2e).

Chlorofluorocarbons (CFCs): A family of inert, nontoxic, and easily liquefied chemicals used in refrigeration, air conditioning, packaging, insulation, or as solvents and aerosol propellants. Because CFCs are not destroyed in the lower atmosphere, they drift into the upper atmosphere, where their chlorine components destroy ozone.

Cogeneration: The generation of electricity and the capture and use of otherwise wasted heat energy byproducts. Cogeneration is also referred to as a combined heat and power (CHP) system.

Combined Heat and Power (CHP): The generation of electricity and the capture and use of otherwise wasted heat energy byproducts. A CHP system is also referred to as cogeneration.

Combined Sewer Overflow (CSO): A combined sewer system conveys both sanitary sewage and stormwater in one piping system. During normal dry weather conditions, sanitary wastewater collected in the combined sewer system is diverted to the wastewater treatment plant before it enters natural waterways. During periods of significant rainfall, the capacity of a combined sewer may be exceeded. When this occurs, excess flow, a mixture of stormwater and sanitary wastewater, is discharged at CSO points, typically to rivers and streams. Release of this excess flow is necessary to prevent flooding in homes, basements, businesses, and streets. Visit the National Pollutant Discharge Elimination System's Combined Sewer Overflows website for additional information.

Cooling Tower Blowdown Water: Water released from a cooling tower to maintain proper water mineral concentration.

Cooling Tower Make-Up Water: Water added to a cooling tower to replace water lost to evaporation or blowdown.


Deionize (DI): To remove ions from a solution using an ion-exchange process.

Delivered Product: Green power fed into the same electric transmission and distribution system that serves the end user. Review the Green Tags vs. Delivered Product (PDF) (3 pp, 233K, About PDF) document to understand the difference between the two green power options: delivered product and renewable energy certificates.

Direct Digital Controls (DDC): The application of microprocessor technology to building environmental controls. DDC systems make it possible to control heating and cooling functions with software that takes into account a wide range of variables, thereby achieving greater efficiency.


Evapotranspiration: The loss of water from the soil both by evaporation and by transpiration from the plants growing in the soil.

Extensive Garden: Extensive gardens have thinner soil depths and require less management and less structural support than intensive gardens. They do not require artificial irrigation. Plants chosen for these gardens are low-maintenance, hardy species that do not have demanding habitat requirements. The goal of an extensive planting design is to have a self-sustaining plant community.


Free Cooling: A method of using ambient air temperatures to create chilled water, which is then used for air conditioning or cooling mechanical systems.


Green Roof: Also known as rooftop gardens, green roofs are planted over existing roof structures, and consist of a waterproof, root-safe membrane that is covered by a drainage system, lightweight growing medium, and plants. Green roofs reduce rooftop and building temperatures, filter pollution, lessen pressure on sewer systems, and reduce the heat island effect.

Green Infrastructure: An adaptable term used to describe an array of products, technologies, and practices that use natural systems – or engineered systems that mimic natural processes – to enhance overall environmental quality and provide utility services. As a general principal, Green Infrastructure techniques use soils and vegetation to infiltrate, evapotranspirate, and/or recycle stormwater runoff.

Green Tags: See renewable energy certificates.

Grey Water: Non-drinkable water that can be reused for irrigation, flushing toilets, and other purposes.

Ground Source Heat Pump (GSHP): Electrically powered systems that tap the stored energy of the earth. These systems use the earth's relatively constant temperature to provide heating, cooling, and hot water for homes and commercial buildings.


Heating, Ventilation, and Air Conditioning (HVAC): Controls the ambient environment (temperature, humidity, air flow, and air filtering) of a building and must be planned for and operated along with other data center components such as computing hardware, cabling, data storage, fire protection, physical security systems, and power.

Heat Island Effect: This phenomenon describes urban and suburban temperatures that are 2° to 10°F (1° to 6°C) warmer than nearby rural areas. For more information, visit EPA's Heat Island website.

Heat Pipe: A device that can quickly transfer heat from one point to another. Heat pipes are often referred to as the "superconductors" of heat, as they possess an extraordinary heat transfer capacity and rate with almost no heat loss.

Heat Recovery System: Process by which the exhaust air preheats the supply air when the outdoor air is cooler than the inside air. When the outside air is warmer than the inside air, the exhaust air will cool the supply air. This system saves energy depending on the season and only operates for sensible heat recovery.

Hydrofluorocarbons (HFCs): Man-made compounds containing hydrogen, fluorine, and carbon, many of which have been developed as alternatives to ozone-depleting substances for industrial, commercial, and consumer products, that have a range of global warming potentials. HFCs do not have the potential to destroy stratospheric ozone, but they are still powerful greenhouse gases.

Hydrochlorofluorocarbons (HCFCs): Compounds containing carbon, hydrogen, chlorine, and fluorine. They were originally intended as replacements for CFCs, but are only a temporary solution because they still contain chlorine and have the potential to destroy stratospheric ozone.


Intensive vs. Extensive Gardens: Intensive gardens have thicker soil depths and generally require more management and artificial irrigation systems. The plants chosen for these gardens must thrive in the specific roof environment they inhabit. Intensive gardens are heavier than extensive gardens, requiring more structural support. Extensive gardens have thinner soil depths and require less management and structural support. They do not require artificial irrigation. Plants chosen for extensive gardens are low maintenance, hardy species and do not have demanding habitat requirements. The idea of an extensive planting design is to have a self-sustaining plant community.

Irrigation: Supplying water to grass, trees, and other plants.


Lab Process Water: Water used for laboratory experiments and procedures.

Low Impact Development (LID): A sustainable landscaping approach that can be used to replicate or restore natural watershed functions and/or address targeted watershed goals and objectives.


Mercury: A naturally occurring element that is found in air, water, and soil. It exists in several forms: elemental or metallic mercury, inorganic mercury compounds, and organic mercury compounds. Mercury is used in thermometers, fluorescent light bulbs, and some electrical switches. For more information, visit EPA's Mercury website.

Methane (CH4): A greenhouse gas that is emitted during the production and transport of coal, natural gas, and oil or from the decomposition of organic wastes in municipal solid waste landfills and the raising of livestock. Although CO2 is more prevalent in the atmosphere, methane is over 20 times more effective in trapping heat than CO2 over a 100-year period. For more information, visit EPA's Methane website.


Net Metering: A method of crediting customers for electricity that they generate on site in excess of their own electricity consumption. Customers with their own generation offset the electricity they would have purchased from their utility. If such customers generate more than they use in a billing period, their electric meter turns backwards to indicate their net excess generation. Depending on individual state or utility rules, the net excess generation may be credited to their account (in many cases at the retail price), carried over to a future billing period, or ignored.

Nitrogen Oxides (NOx): A group of gases that cause acid rain and other environmental problems, such as smog and eutrophication of coastal waters. Burning fossil fuels such as coal and gasoline releases NOx into the atmosphere. For more information, visit EPA's Air and Radiation website.

Nitrous Oxide (N2O): A clear, colorless gas, with slightly sweet odor. Due to its long atmospheric lifetime (approximately 120 years) and heat trapping effects—about 310 times more powerful than carbon dioxide on a per molecule basis—N2O is a greenhouse gas that contributes to global climate change.


Onsite Renewable Energy Generation: Electricity generated by renewable resources using a system or device located at the site where the power is used. Onsite generation is a form of distributed energy generation.

Operations and Maintenance (O&M): The activities related to the performance of routine, preventive, predictive, scheduled, and unscheduled actions aimed at preventing equipment failure or decline with the goal of increasing efficiency, reliability, and safety.


Particulate Matter: A complex mixture of extremely small particles and liquid droplets. It is made up of a number of components, including acids (such as nitrates and sulfates), organic chemicals, metals, and soil or dust particles.

Perfluorocarbons (PFCs): Potent greenhouse gases that accumulate in the atmosphere and remain there for thousands of years. Aluminum production and semiconductor manufacture are the largest known man-made sources of perfluorocarbons. For more information, visit EPA's High Global Warming Potential Gases and Climate Change page.

Photovoltaic (PV): A system that converts sunlight directly into electricity using cells made of silicon or other conductive materials. When sunlight hits the cells, a chemical reaction occurs, resulting in the release of electricity.

Predevelopment Hydrology: The combination of runoff, infiltration, and evapotranspiration rates and volumes that typically existed on a site before human-induced land disturbance occurred (e.g., construction of infrastructure on undeveloped land such as meadows or forests).


R Value: A measure of insulation. The higher the R-value, the better walls and roofs will resist the transfer of heat. For more information visit the Department of Energy's website.

Rain Chains: A water feature that is used as an alternative to a downspout. Rain chains guide runoff from a roof to either the ground, a cistern, or rain barrel. 

Rain Garden: A rain garden is a depressed area of the ground planted with vegetation, allowing runoff from impervious surfaces such as parking lots and roofs the opportunity to be collected and infiltrated into the groundwater supply or returned to the atmosphere through evaporation and evapotranspiration.

Renewable Energy Certificates (RECs): Also known as green tags, green energy certificates, or tradable renewable certificates, RECs represent the technology and environmental attributes of electricity generated from renewable sources. Renewable energy certificates are usually sold in 1 megawatt-hour (MWh) units. A certificate can be sold separately from the MWh of generic electricity it is associated with. This flexibility enables customers to offset a percentage of their annual electricity use with certificates generated elsewhere. For more information, view Green Tags vs. Delivered Product (PDF) (3 pp, 233K, About PDF).

Renewable Energy Credits: See renewable energy certificates.

Reverse Osmosis (RO): A membrane separation process designed to treat wastewater containing a variety of contaminants including organic compounds.

Reverse Osmosis Reject Water: Waste water released from the reverse osmosis process.


Sanitary Water Use: Domestic water uses, such as faucets, toilets, and showers.

Scope 1 GHG Emissions: Direct GHG emissions from sources that are owned or controlled by the reporting entity. This can include emissions from fossil fuels burned on site, emissions from agency-owned or agency-leased vehicles, and other direct sources.

Scope 2 GHG Emissions: Indirect GHG emissions resulting from the generation of electricity, heat, or steam generated off site but purchased by the reporting agency.

Scope 3 GHG Emissions: Indirect GHG emissions from sources not owned or directly controlled by the reporting agency but related to the agency’s activities such as vendor supply chains, delivery services, outsourced activities, and employee travel and commuting.

Single-Pass Cooling: During this process, water is circulated once through a piece of equipment and then disposed down the drain. To remove the same heat load, single-pass systems use 40 times more water than a cooling tower operated at 5 cycles of concentration.

Stormwater Infiltration: The process through which stormwater runoff penetrates into soil from the ground surface.

Sulfur Dioxide (SO2): A gas that causes acid rain. Burning fossil fuels, such as coal, releases SO2 into the atmosphere. For more information, visit EPA's Air and Radiation website.

Sulfur Hexafluoride (SF6): A potent greenhouse gas primarily used as an electrical insulator in high voltage equipment. For more information, visit EPA's SF6 Emission Reduction Partnership for Electric Power Systems page.


Telework: A flexible work arrangement where a manager allows an employee to perform his or her professional duties and responsibilities at an approved alternative worksite during any part of the employee's regular, paid hours. For more information visit the U.S. Office of Personnel Management's Telework Frequently Asked Questions website.

Touchdown Station: A workspace that is used as a substitute for a larger office environment. These stations are typically small, open-air cubicles with access to basic business essentials (i.e., Internet access, office chair, phone line), and can also include shared meeting space.

Tradable Renewable Credits: See renewable energy certificates.


Variable Air Volume (VAV): An HVAC system strategy through which the volume of air delivered to conditioned spaces is varied as a function of ventilating needs, energy needs, or both.

Variable Frequency Drive (VFD): A specific type of adjustable-speed drive that controls the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. VFDs are also known as adjustable-frequency drives (AFD), variable-speed drives (VSD), AC drives, or inverter drives.

Volatile Organic Compounds (VOCs): Emitted as gases from certain solids or liquids, VOCs include substances—some of which may have short- and long-term adverse health effects—such as benzene, toluene, methylene chloride, and methyl chloroform.


Water Side Economizer: A water side economizer reduces energy consumption in cooling mode by allowing the chiller to be turned off when the cooling tower alone can produce water at the desired chilled water set point. The cooling tower system, rather than the chiller, provides the cold water for cooling.

Wet Laboratory: Laboratory where chemicals, drugs, or other material or biological matter is tested and analyzed, and which requires water, direct ventilation, and specialized piped utilities.

Wet Weather Green Infrastructure: Infrastructure associated with stormwater management and low impact development that encompasses approaches and technologies to infiltrate, evapotranspire, capture, and reuse stormwater to maintain or restore natural hydrologies.


Xeriscaping: Landscaping based on native, water-efficient plants to minimize the need for irrigation.

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