ISM bands are slices of spectrum that regulators let anyone use without an individual license, provided equipment meets power limits and, in some regions, duty cycle or listen-before-talk rules. The best known are 2.4 GHz, shared by Wi-Fi, Bluetooth and Zigbee worldwide, and the sub-GHz bands around 868 MHz in Europe and 915 MHz in North America used by LoRa, Sigfox and Z-Wave.
For IoT this is the foundation of most non-cellular connectivity: no spectrum fees, no operator contract, and hardware that can be sold globally with regional variants. The price is interference. Nobody has priority, so a crowded 2.4 GHz band in an apartment block or a factory full of welders can degrade a network, and products must still pass certification such as FCC or CE before sale.
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