Definition - What does Quad Band mean?
Quad band is a device feature that supports four different frequency bands used in communication: 850 MHz, 900 MHz, 1,800 MHz and 1,900 MHz.
In the context of mobile phones, a quad band feature gives the user wider roaming capabilities. A quad band phone running on the global system for mobile communications (GSM) network will be able to roam anywhere in the world where GSM service is available. This is an important feature because each GSM network uses different frequency bands.
Techopedia explains Quad Band
Europe uses the 900 and 1,800 bands, while the U.S. uses the 850 and 1,900 bands. Thus, if the user lives in the U.S. and owns a phone that only operates on one frequency band, the phone will not work abroad. Even if the phone supports dual bands, if those bands are just the 850 and 1,900 bands, the phone will not be usable in Europe.