For the numbers used to describe data centres (megawatts, IT load, PUE, WUE and more), see Data-centre numbers explained.
- AI Growth Zone
- An area designated by the UK Government to attract AI data-centre investment, with support on planning and power. South Wales has one.
- Backup (standby) generators
- Diesel or gas generators that keep a data centre running during a power cut. They are tested regularly and produce emissions when they run.
- Call-in
- When ministers take a planning decision away from the local council and decide it themselves. A holding direction pauses a decision while they consider this.
- CLOUD Act
- A 2018 United States law under which US authorities can, in some circumstances, require US-based companies to provide data they control, even if it is stored abroad.
- Cooling (air, evaporative, liquid, immersion)
- Ways of removing heat from computers. Evaporative cooling uses water directly; air cooling uses more electricity; liquid and immersion cooling bring fluid closer to the chips, and some immersion fluids are PFAS.
- Critical National Infrastructure (CNI)
- Assets the Government considers essential to the country. Data infrastructure was designated CNI in September 2024.
- Data residency / data sovereignty
- Residency is where data is physically stored. Sovereignty is whose laws and authorities ultimately control it. They are not the same.
- Environmental Impact Assessment (EIA)
- A formal process for assessing a large development’s likely significant environmental effects before a decision is made. A screening opinion decides whether one is needed.
- Environmental Information Regulations (EIR)
- The law giving you a right to environmental information held by public authorities — including water companies. Similar to freedom of information, often with fewer exemptions.
- Freedom of Information (FOI)
- Your right to ask public bodies for recorded information. Replies are normally due within 20 working days.
- Grid connection
- The agreement allowing a site to draw a set amount of power from the network. Large connections may need new substations and lines.
- Hyperscale
- A very large data centre or campus, typically run by one of the major cloud companies.
- MW, GW and TWh
- Megawatts and gigawatts measure capacity (how much power can be drawn at once; 1 GW = 1,000 MW). Terawatt-hours measure energy used over time.
- PFAS (‘forever chemicals’)
- Per- and polyfluoroalkyl substances: a large family of man-made chemicals that barely break down in nature. Some fire-suppression gases and cooling fluids used in data centres are PFAS.
- Planning Policy Wales / NPPF
- The main national planning policy documents for Wales and for England respectively.
- Pre-application consultation
- In Wales, developers of major schemes must consult the community before submitting a planning application.
- Precautionary principle
- Where there is a threat of serious or irreversible harm, lack of full scientific certainty should not be a reason to delay action to prevent it.
- Water stress
- When demand for water is high relative to the amount reliably available. The Environment Agency classifies water-stressed areas in England.
- Well-being of Future Generations (Wales) Act 2015
- Welsh law requiring public bodies to act in a way that does not compromise the ability of future generations to meet their own needs.
- WhatDoTheyKnow
- A free website for making freedom of information requests; requests and replies are published so everyone can see them.