What does uniterruptible power supply mean?

An uninterruptable power supply is a power device for a computer (or other things) which is designed never to lose power in the middle of crucial operations. For example, if you're running a computer which is in charge of a critical process and there is a disruption in the power coming into the computer, the uninterruptable PSU (power supply unit) is designed to make sure the computer doesn't stop the process it was working on and lose data.