Uninterruptible Power Supply
An Uninterruptible Power Supply is a device that provides battery backup when the electrical power fails or drops to an unacceptable voltage level. Small UPS systems provide power for a few minutes; enough to power down the computer in an orderly manner, while larger systems have enough battery for several hours. In mission critical datacenters, UPS systems are used for just a few minutes until electrical generators take over.
UPS systems can be set up to alert file servers to shut down in an orderly manner when an outage has occurred, and the batteries are running out.