If an electronic device can be operated by either battery or mains electricity, what would it be considered?

Dual voltage.

I believe the question is asking for a term to describe a device that can operate on both AC power or a battery backup, such as an alarm clock which uses the battery as an alternate power source if the electricity goes out. I thought "dual voltage" was the term used for electrical devices that can operate on both 110 or 220 current. Here's an example:

Most laptop computers and a growing array of other travel oriented devices are now "dual voltage" meaning they are designed to operate safely with either 110/120V or 220/240V systems. If a device is dual voltage, all that will be needed to operate it in a foreign country is an electric adapter plug, to convert the device's electric plug to the style of foreign electric socket. If a device is not dual voltage, it may need a converter or transformer to actually change the voltage it uses in the foreign country.
https://www.mousetourstravels.com/dual-voltage-appliances.html