ISM means "Industrial, Scientific, Medical".

In order to avoid caotic interference of electromagnetic signals an international standardisation defines frequency ranges which may be used for industrial, scientific and medical purposes. These reserved frequencies are 40 kHz, 27,12 MHz, 13,56 MHz, 2,45 GHz. 

High Frequency
HF(High frequency)-Plasma generators can use the folllowing ISM frequencies: 40 kHz, 13,56 MHz, 27,12 MHz or 2,45 GHz.