Data Center Glossary A–Z
Plain-language definitions for 51 data center acronyms, measurements and operating concepts used across the Operator Academy and Data Center Fan.
- A/B feed
- Two intended independent power paths serving dual-corded equipment. Independence must continue upstream.
- AHU
- Air handling unit: equipment that moves and conditions air; the exact components depend on the design.
- ATS
- Automatic transfer switch: transfers a load between eligible electrical sources when control conditions are met.
- Availability
- The proportion of a defined observation period during which a service is usable according to its criteria.
- BMS
- Building management system: supervises building and mechanical controls, points, trends and alarms.
- Bypass air
- Conditioned air that returns without passing through the IT equipment it was meant to cool.
- Centrifugal chiller
- A vapor-compression chiller using a centrifugal compressor to move heat between water loops.
- Cold aisle
- The aisle facing equipment air inlets, supplied with conditioned air in a front-to-front rack arrangement.
- Concurrent maintainability
- Ability to remove a planned component or path from service without interrupting the critical load.
- Containment
- Physical separation that reduces mixing between cooling supply air and hot equipment exhaust.
- COP
- Coefficient of performance: heat moved divided by electrical power consumed by the cooling process.
- CRAC
- Computer room air conditioner, commonly a room-air unit with direct-expansion refrigeration.
- CRAH
- Computer room air handler, commonly a room-air unit using chilled water from a central plant.
- CUE
- Carbon usage effectiveness: a metric relating carbon emissions to IT equipment energy.
- DCIM
- Data center infrastructure management: tools joining facility capacity, environmental, power and asset information.
- Delta T
- Temperature difference between two measurement points, often supply and return or equipment inlet and exhaust.
- Direct-to-chip
- Liquid cooling that transfers heat from major components through cold plates connected to a fluid loop.
- Economizer
- A mode using favorable outdoor conditions to reduce or avoid mechanical refrigeration work.
- EPMS
- Electrical power monitoring system: metering, event capture and analysis for the electrical distribution system.
- Fault domain
- A set of resources expected to be affected by the same underlying failure.
- Free cooling
- Reduction of compressor-based cooling by using suitable ambient air or water conditions; it is not energy-free.
- Hot aisle
- The aisle receiving equipment exhaust in a back-to-back rack arrangement.
- HPC
- High-performance computing: tightly coupled compute systems optimized for large scientific or technical workloads.
- Immersion cooling
- Cooling in which compatible IT equipment is placed in a dielectric fluid that carries away heat.
- Inlet temperature
- Air temperature entering IT equipment; measure at the equipment inlet, not only at the room sensor.
- IT load
- Electrical demand of servers, storage and network equipment inside the PUE measurement boundary.
- kW
- Kilowatt: a unit of power equal to 1,000 watts, describing the instantaneous rate of energy use.
- kWh
- Kilowatt-hour: a unit of energy equal to one kilowatt sustained for one hour.
- Latency
- Delay between a request or signal and its response, often measured one-way or round-trip.
- MAH
- Make-up air handler: supplies outdoor air to replace exhausted air and support pressure or ventilation needs.
- MTBF
- Mean time between failures: average operating time between repairable failure events under a stated definition.
- MTTR
- Mean time to repair or restore; teams must state which meaning and which clock boundaries they use.
- N
- The capacity or component count required to carry the defined design load.
- N+1
- The required N capacity plus one additional usable component of the relevant size.
- Network fabric
- Interconnected switching topology that carries traffic between endpoints and upstream networks.
- PDU
- Power distribution unit: distributes electrical power to downstream panels, circuits or rack equipment.
- Power density
- Electrical load per rack, area or other physical unit, commonly expressed as kW per rack.
- PUE
- Power usage effectiveness: total data center energy or power divided by IT equipment energy or power.
- Rack PDU
- Outlet-level distribution device installed in a rack and fed by a branch circuit.
- Rear-door heat exchanger
- A liquid-cooled rack door that captures heat from server exhaust.
- Recirculation
- Hot exhaust air that returns to an IT equipment inlet instead of reaching the cooling return path.
- RPO
- Recovery point objective: the maximum targeted amount of data loss measured backward in time.
- RTO
- Recovery time objective: the targeted time to restore a service after disruption.
- SLA
- Service-level agreement: a documented service commitment, measurement method and consequence or remedy.
- Single point of failure
- One component, path or process whose failure can interrupt the protected service by itself.
- Static transfer switch
- Solid-state device that rapidly transfers eligible loads between compatible AC sources.
- UPS
- Uninterruptible power supply: provides immediate stored energy and power conditioning to protected loads.
- WUE
- Water usage effectiveness: site water use divided by IT equipment energy, commonly in liters per kWh.
- 2N
- Two complete sets of capacity, each independently able to carry the defined N load.
- 2N+1
- Two complete N systems plus an additional redundant component; exact interpretation must be documented.
- 1U
- One rack unit of vertical mounting height, equal to 1.75 inches or 44.45 millimeters.