Communications engineering. The ISM band contains several frequency ranges which can be used licence-free (i.e. without further permission through the authorities) by everybody. Only the maximum transmission power and the interference with other frequency bands are regulated. The ISM band is not regulated consistently world wide, there are differences from country to country in regard to frequency ranges as well as to transmission power. Originally it was defined for industrial, scientific and medical use, not for communications.
A frequency range used by many applications, for example ZigBee, Bluetooth, WLans, RFID in the microwave area is the 2.4 GHz band, described in EN 300328.