Tri-mode designates a phone that supports three specific combinations of frequency band and technology (modes).
Currently, the most common type of tri-mode phone in North America is one that supports CDMA (digital) in both the 850 and 1900 MHz bands, and AMPS (analog) in the 850 band.
Most TDMA phones were also tri-mode, supporting TDMA (digital) in both the 850 and 1900 MHz bands, and AMPS (analog) in the 850 band.
Tri-mode is not be confused with tri-band. Tri-mode usually refers to a CDMA phone, while tri-band usually refers to GSM phones.