Final answer:
A UPS (Uninterruptible Power Supply) is used to provide reliable power supply to server computers.
Step-by-step explanation:
The device used to provide a reliable power supply to server computers is called a UPS (Uninterruptible Power Supply). A UPS is an electrical device that provides temporary power to a computer system during a power outage or when there is a fluctuation in the main power supply. It consists of a battery and an inverter that converts the battery power into AC power to keep the server running.