Frequency bands allocated internationally for Industrial, Scientific, and Medical (ISM) applications are centered at various frequencies between 13 and 5700 MHz. They're unregulated, but before ISM equipment can be marketed in the US, it still must comply with the technical standards and equipment authorization procedures specified in the FCC's Part 18. Since they operate in the same spectral neighborhood as systems used for commercial communications, ISM products must accept the possibility of being the recipient of interference from them, but cannot generate interference to these systems.
ISM applications are most commonly considered as any non-commercial use that does not result in the transmission of a signal for communication purposes. While this is a nice, compact definition (and one that may once have been true), it is accurate no longer, since both Bluetooth and WiFi operate in 2.4-GHz ISM bands. WiFi has evolved from its origin as a "wireless LAN" within an enterprise to a way to draw people to a business location and keep them there longer. So the "non-commercial" moniker no longer applies.
While the acronym ISM refers to specific frequency bands, the industrial marketplace for RF power devices extends outside the ISM band for applications such as RF heating, HF and VHF communications, and magnetic resonance imaging (MRI). All of these applications require high RF power levels, and have been served by solid-state devices for many years. While these many applications bear little similarity, they share the need for devices that are efficient, produce large amounts of gain, and are cost-effective, rugged, and can be manufactured in large quantities.
However, nearly all applications that operate in the ISM bands, as well as those operating at frequencies below 450 MHz, are truly non-commercial and include RF heating for manufacturing purposes, plasma generators, laser exciters, RF lighting, medical telemetry, and many, many others. In some of the applications, RF output power is only a few milliwatts, while in others (such as RF heating and MRI) power levels can be hundreds or thousands of watts.