The term Subradio is sometimes used for Electromagnetic Radiation with Frequencies below that of Radio. In communications, such frequencies are used for only very specialized applications, and the "lower end of radio frequencies" is not universally agreed upon, and has been placed at different frequencies, e.g., 3Hz, 3kHz, or 30kHz.
Bands at the lower end of radio or beyond, i.e., subradio:
Extremely Low Frequency (ELF)
Super Low Frequency (SLF)
Tremendously Low Frequency (TLF)
Ultra Low Frequency (ULF)
Very Low Frequency (VLF)