The term "battery backup" refers to a device or system that provides power in case of a main power source failure. This type of backup is typically used for electronic devices, such as computers and servers, to ensure they can continue functioning during a power outage. The battery backup is designed to provide enough energy to keep the device running until the power comes back on, or until the battery runs out. In essence, it acts as an insurance policy against unexpected power interruptions that could cause data loss or equipment damage.