A designation indicating that a phone supports two different frequency bands. Not meaningful without knowing which two bands and which technologies the phone will work with.
For North American TDMA and CDMA phones, dual-band indicates that the phone will work in both the 800/850 MHz band and the 1900 MHz band. Most current TDMA phones in the U.S. are tri-mode, meaning they can use both analog and digital in the 800/850 band. Dual-band CDMA phones can be dual-mode or tri-mode. Dual-mode phones only support analog or digital on the 800/850 band, while tri-mode phones support both. The 1900 MHz band is always digital.
A dual-band GSM phone supports two of the four major GSM bands. Depending on which bands, the phone may only work in certain parts of the world. A GSM 850/1900 phone will only in the Americas. A GSM 900/1800 phone will only work in Europe, Africa, Asia, and Brazil. A GSM 900/1900 phone will work on at least one network in most countries around the world.
NOTE: This definition applies to the U.S. and Canada, and may not be accurate for other countries.