ISM means Industrial, Scientific and Medical frequency band. This is a band of radio and microwave frequencies clustered around 2.4GHz, reserved and designated for industrial, scientific and medical equipment that use RF. Industrial equipment like MRI machines, testing equipment, and some radio telescopes use this ISM band of frequencies. Smaller consumer devices such as microwave ovens, garage door system, codeless phone, wireless router and wireless mouse are also designed to run at frequencies around 2.4 GHz.
Most telecommunication devices operate at a much lower frequency than 2.4GHz. Having other devices operate at only a certain frequency range, in this case, ISM, reduces interference with the telecommunication frequencies. This means that using a wireless router and a cellular phone at the time will have no interference with each other.
2.4GHz is not the only ISM frequency. It is only the most common because there is no need for licensing devices to use it. Other ISM frequencies can be as high as 24.125GHz or even as low as 13.56MHz. Depending on the location and local acceptance, authorities can allocate ISM radio frequencies with some little flexibility.
Allocation of ISM radio frequencies is stipulated by the International Telecommunication Union (ITU). ITU has documented a worldwide ISM allocation table which varies slightly depending on the region. ISM users must accept laid down terms and regulations to ensure safety and avoid interference.