Refers to an antenna, communication device, or other RF hardware that works at two different frequencies (or bands).
International mobile communication networks use different frequencies. Unlike earlier models, German mobile phones today also work on the European network i.e. two networks; hence the name dual band. (see also Tri-band)
A handset that works on both the 800 MHz cellular and the 1900 MHz PCS frequencies in North America (900MHz cellular and 1800 MHz PCS in Europe).